programming python

Tuổi thọ python 3.9

Python is the main dynamic language used at Google. This style guide is a list of dos and don’ts for Python programs

To help you format code correctly, we’ve created a settings file for Vim. For Emacs, the default settings should be fine

Many teams use the yapf auto-formatter to avoid arguing over formatting

2 Python Language Rules

2. 1 Lint

Run

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

18 over your code using this pylintrc

2. 1. 1 Definition

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

18 is a tool for finding bugs and style problems in Python source code. It finds problems that are typically caught by a compiler for less dynamic languages like C and C++. Because of the dynamic nature of Python, some warnings may be incorrect; however, spurious warnings should be fairly infrequent

2. 1. 2 Pros

Catches easy-to-miss errors like typos, using-vars-before-assignment, etc

2. 1. 3 Cons

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

18 isn’t perfect. To take advantage of it, sometimes we’ll need to write around it, suppress its warnings or fix it

2. 1. 4 Decision

Make sure you run

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

18 on your code

Suppress warnings if they are inappropriate so that other issues are not hidden. To suppress warnings, you can set a line-level comment

dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

18 warnings are each identified by symbolic name [

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

23] Google-specific warnings start with

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

If the reason for the suppression is not clear from the symbolic name, add an explanation

Suppressing in this way has the advantage that we can easily search for suppressions and revisit them

You can get a list of

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

18 warnings by doing

To get more information on a particular message, use

Prefer

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

26 to the deprecated older form

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

Unused argument warnings can be suppressed by deleting the variables at the beginning of the function. Always include a comment explaining why you are deleting it. “Unused. ” is sufficient. For example

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

Other common forms of suppressing this warning include using ‘

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

28’ as the identifier for the unused argument or prefixing the argument name with ‘

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

29’, or assigning them to ‘

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

28’. These forms are allowed but no longer encouraged. These break callers that pass arguments by name and do not enforce that the arguments are actually unused

2. 2 Imports

Use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

31 statements for packages and modules only, not for individual classes or functions

2. 2. 1 Definition

Reusability mechanism for sharing code from one module to another

2. 2. 2 Pros

The namespace management convention is simple. The source of each identifier is indicated in a consistent way;

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

32 says that object

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

33 is defined in module

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

2. 2. 3 Cons

Module names can still collide. Some module names are inconveniently long

2. 2. 4 Decision

Use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

35 for importing packages and modules

Use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

36 where

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

34 is the package prefix and

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

38 is the module name with no prefix

Use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

39 if two modules named

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

38 are to be imported, if

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

38 conflicts with a top-level name defined in the current module, or if

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

38 is an inconveniently long name

Use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

43 only when

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

44 is a standard abbreviation [e. g. ,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

45 for

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

46]

For example the module

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

47 may be imported as follows

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

Do not use relative names in imports. Even if the module is in the same package, use the full package name. This helps prevent unintentionally importing a package twice

2. 2. 4. 1 Exemptions

Exemptions from this rule

Symbols from the following modules are used to support static analysis and type checking
Redirects from the six. moves module

2. 3 Packages

Import each module using the full pathname location of the module

2. 3. 1 Pros

Avoids conflicts in module names or incorrect imports due to the module search path not being what the author expected. Makes it easier to find modules

2. 3. 2 Cons

Makes it harder to deploy code because you have to replicate the package hierarchy. Not really a problem with modern deployment mechanisms

2. 3. 3 Decision

All new code should import each module by its full package name

Imports should be as follows

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

[assume this file lives in

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

48 where

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

49 also exists]

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

The directory the main binary is located in should not be assumed to be in

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

50 despite that happening in some environments. This being the case, code should assume that

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

51 refers to a third party or top level package named

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

52, not a local

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

2. 4 Exceptions

Exceptions are allowed but must be used carefully

2. 4. 1 Definition

Exceptions are a means of breaking out of normal control flow to handle errors or other exceptional conditions

2. 4. 2 Pros

The control flow of normal operation code is not cluttered by error-handling code. It also allows the control flow to skip multiple frames when a certain condition occurs, e. g. , returning from N nested functions in one step instead of having to plumb error codes through

2. 4. 3 Cons

May cause the control flow to be confusing. Easy to miss error cases when making library calls

2. 4. 4 Decision

Exceptions must follow certain conditions

Make use of built-in exception classes when it makes sense. For example, raise a

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

54 to indicate a programming mistake like a violated precondition [such as if you were passed a negative number but required a positive one]. Do not use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

55 statements for validating argument values of a public API.

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

55 is used to ensure internal correctness, not to enforce correct usage nor to indicate that some unexpected event occurred. If an exception is desired in the latter cases, use a raise statement. For example

dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin

dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin

Libraries or packages may define their own exceptions. When doing so they must inherit from an existing exception class. Exception names should end in

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

57 and should not introduce repetition [

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

58]

Never use catch-all

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

59 statements, or catch

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

60 or

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

61, unless you are

re-raising the exception, or
creating an isolation point in the program where exceptions are not propagated but are recorded and suppressed instead, such as protecting a thread from crashing by guarding its outermost block

Python is very tolerant in this regard and

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

59 will really catch everything including misspelled names, sys. exit[] calls, Ctrl+C interrupts, unittest failures and all kinds of other exceptions that you simply don’t want to catch

Minimize the amount of code in a

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

63/

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

64 block. The larger the body of the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

63, the more likely that an exception will be raised by a line of code that you didn’t expect to raise an exception. In those cases, the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

63/

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

64 block hides a real error

Use the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

68 clause to execute code whether or not an exception is raised in the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

63 block. This is often useful for cleanup, i. e. , closing a file

2. 5 Mutable Global State

Avoid mutable global state

2. 5. 1 Definition

Module level values or class attributes that can get mutated during program execution

2. 5. 2 Pros

Occasionally useful

2. 5. 3 Cons

Breaks encapsulation. Such design can make it hard to achieve valid objectives. For example, if global state is used to manage a database connection, then connecting to two different databases at the same time [such as for computing differences during a migration] becomes difficult. Similar problems easily arise with global registries
Has the potential to change module behavior during the import, because assignments to global variables are done when the module is first imported

2. 5. 4 Decision

Avoid mutable global state

In those rare cases where using global state is warranted, mutable global entities should be declared at the module level or as a class attribute and made internal by prepending an

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

28 to the name. If necessary, external access to mutable global state must be done through public functions or class methods. See Naming below. Please explain the design reasons why mutable global state is being used in a comment or a doc linked to from a comment

Module-level constants are permitted and encouraged. For example.

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

71 for an internal use constant or

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

72 for a public API constant. Constants must be named using all caps with underscores. See Naming below

2. 6 Nested/Local/Inner Classes and Functions

Nested local functions or classes are fine when used to close over a local variable. Inner classes are fine

2. 6. 1 Definition

A class can be defined inside of a method, function, or class. A function can be defined inside a method or function. Nested functions have read-only access to variables defined in enclosing scopes

2. 6. 2 Pros

Allows definition of utility classes and functions that are only used inside of a very limited scope. Very ADT-y. Commonly used for implementing decorators

2. 6. 3 Cons

Nested functions and classes cannot be directly tested. Nesting can make the outer function longer and less readable

2. 6. 4 Decision

They are fine with some caveats. Avoid nested functions or classes except when closing over a local value other than

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

73 or

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

74. Do not nest a function just to hide it from users of a module. Instead, prefix its name with an _ at the module level so that it can still be accessed by tests

2. 7 Comprehensions & Generator Expressions

Okay to use for simple cases

2. 7. 1 Definition

List, Dict, and Set comprehensions as well as generator expressions provide a concise and efficient way to create container types and iterators without resorting to the use of traditional loops,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

75,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

76, or

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

2. 7. 2 Pros

Simple comprehensions can be clearer and simpler than other dict, list, or set creation techniques. Generator expressions can be very efficient, since they avoid the creation of a list entirely

2. 7. 3 Cons

Complicated comprehensions or generator expressions can be hard to read

2. 7. 4 Decision

Okay to use for simple cases. Each portion must fit on one line. mapping expression,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

78 clause, filter expression. Multiple

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

78 clauses or filter expressions are not permitted. Use loops instead when things get more complicated

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

2. 8 Default Iterators and Operators

Use default iterators and operators for types that support them, like lists, dictionaries, and files

2. 8. 1 Definition

Container types, like dictionaries and lists, define default iterators and membership test operators [“in” and “not in”]

2. 8. 2 Pros

The default iterators and operators are simple and efficient. They express the operation directly, without extra method calls. A function that uses default operators is generic. It can be used with any type that supports the operation

2. 8. 3 Cons

You can’t tell the type of objects by reading the method names [unless the variable has type annotations]. This is also an advantage

2. 8. 4 Decision

Use default iterators and operators for types that support them, like lists, dictionaries, and files. The built-in types define iterator methods, too. Prefer these methods to methods that return lists, except that you should not mutate a container while iterating over it

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

2. 9 Generators

Use generators as needed

2. 9. 1 Definition

A generator function returns an iterator that yields a value each time it executes a yield statement. After it yields a value, the runtime state of the generator function is suspended until the next value is needed

2. 9. 2 Pros

Simpler code, because the state of local variables and control flow are preserved for each call. A generator uses less memory than a function that creates an entire list of values at once

2. 9. 3 Cons

Local variables in the generator will not be garbage collected until the generator is either consumed to exhaustion or itself garbage collected

2. 9. 4 Decision

Fine. Use “Yields. ” rather than “Returns. ” in the docstring for generator functions

If the generator manages an expensive resource, make sure to force the clean up

A good way to do the clean up is by wrapping the generator with a context manager PEP-0533

2. 10 Lambda Functions

Okay for one-liners. Prefer generator expressions over

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

75 or

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

76 with a

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

2. 10. 1 Definition

Lambdas define anonymous functions in an expression, as opposed to a statement

2. 10. 2 Pros

Convenient

2. 10. 3 Cons

Harder to read and debug than local functions. The lack of names means stack traces are more difficult to understand. Expressiveness is limited because the function may only contain an expression

2. 10. 4 Decision

Okay to use them for one-liners. If the code inside the lambda function is longer than 60-80 chars, it’s probably better to define it as a regular nested function

For common operations like multiplication, use the functions from the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

83 module instead of lambda functions. For example, prefer

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

84 to

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

2. 11 Conditional Expressions

Okay for simple cases

2. 11. 1 Definition

Conditional expressions [sometimes called a “ternary operator”] are mechanisms that provide a shorter syntax for if statements. For example.

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

2. 11. 2 Pros

Shorter and more convenient than an if statement

2. 11. 3 Cons

May be harder to read than an if statement. The condition may be difficult to locate if the expression is long

2. 11. 4 Decision

Okay to use for simple cases. Each portion must fit on one line. true-expression, if-expression, else-expression. Use a complete if statement when things get more complicated

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

2. 12 Default Argument Values

Okay in most cases

2. 12. 1 Definition

You can specify values for variables at the end of a function’s parameter list, e. g. ,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

87. If

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

88 is called with only one argument,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

89 is set to 0. If it is called with two arguments,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

89 has the value of the second argument

2. 12. 2 Pros

Often you have a function that uses lots of default values, but on rare occasions you want to override the defaults. Default argument values provide an easy way to do this, without having to define lots of functions for the rare exceptions. As Python does not support overloaded methods/functions, default arguments are an easy way of “faking” the overloading behavior

2. 12. 3 Cons

Default arguments are evaluated once at module load time. This may cause problems if the argument is a mutable object such as a list or a dictionary. If the function modifies the object [e. g. , by appending an item to a list], the default value is modified

2. 12. 4 Decision

Okay to use with the following caveat

Do not use mutable objects as default values in the function or method definition

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

2. 13 Properties

Properties may be used to control getting or setting attributes that require trivial computations or logic. Property implementations must match the general expectations of regular attribute access. that they are cheap, straightforward, and unsurprising

2. 13. 1 Definition

A way to wrap method calls for getting and setting an attribute as a standard attribute access

2. 13. 2 Pros

Allows for an attribute access and assignment API rather than getter and setter method calls
Can be used to make an attribute read-only
Allows calculations to be lazy
Provides a way to maintain the public interface of a class when the internals evolve independently of class users

2. 13. 3 Cons

Can hide side-effects much like operator overloading
Can be confusing for subclasses

2. 13. 4 Decision

Properties are allowed, but, like operator overloading, should only be used when necessary and match the expectations of typical attribute access; follow the getters and setters rules otherwise

For example, using a property to simply both get and set an internal attribute isn’t allowed. there is no computation occurring, so the property is unnecessary [make the attribute public instead]. In comparison, using a property to control attribute access or to calculate a trivially derived value is allowed. the logic is simple and unsurprising

Properties should be created with the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

91 decorator. Manually implementing a property descriptor is considered a power feature

Inheritance with properties can be non-obvious. Do not use properties to implement computations a subclass may ever want to override and extend

2. 14 True/False Evaluations

Use the “implicit” false if at all possible

2. 14. 1 Definition

Python evaluates certain values as

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

92 when in a boolean context. A quick “rule of thumb” is that all “empty” values are considered false, so

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

93 all evaluate as false in a boolean context

2. 14. 2 Pros

Conditions using Python booleans are easier to read and less error-prone. In most cases, they’re also faster

2. 14. 3 Cons

May look strange to C/C++ developers

2. 14. 4 Decision

Use the “implicit” false if possible, e. g. ,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

94 rather than

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

95. There are a few caveats that you should keep in mind though

Always use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

96 [or

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

97] to check for a

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

98 value. E. g. , when testing whether a variable or argument that defaults to

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

98 was set to some other value. The other value might be a value that’s false in a boolean context

Never compare a boolean variable to

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

92 using

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

201. Use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

202 instead. If you need to distinguish

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

92 from

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

98 then chain the expressions, such as

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

205

For sequences [strings, lists, tuples], use the fact that empty sequences are false, so

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

206 and

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

207 are preferable to

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

208 and

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

209 respectively

When handling integers, implicit false may involve more risk than benefit [i. e. , accidentally handling

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

98 as 0]. You may compare a value which is known to be an integer [and is not the result of

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

211] against the integer 0

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

Note that

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

212 [i. e. ,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

213 as string] evaluates to true

Note that Numpy arrays may raise an exception in an implicit boolean context. Prefer the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

214 attribute when testing emptiness of a

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

215 [e. g.

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

216]

2. 16 Lexical Scoping

Okay to use

2. 16. 1 Definition

A nested Python function can refer to variables defined in enclosing functions, but cannot assign to them. Variable bindings are resolved using lexical scoping, that is, based on the static program text. Any assignment to a name in a block will cause Python to treat all references to that name as a local variable, even if the use precedes the assignment. If a global declaration occurs, the name is treated as a global variable

An example of the use of this feature is

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

2. 16. 2 Pros

Often results in clearer, more elegant code. Especially comforting to experienced Lisp and Scheme [and Haskell and ML and …] programmers

2. 16. 3 Cons

Can lead to confusing bugs. Such as this example based on PEP-0227

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

217 will print

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

218, not

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

219

2. 16. 4 Decision

Okay to use

2. 17 Function and Method Decorators

Use decorators judiciously when there is a clear advantage. Avoid

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

220 and limit use of

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

221

2. 17. 1 Definition

Decorators for Functions and Methods [a. k. a “the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

222 notation”]. One common decorator is

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

91, used for converting ordinary methods into dynamically computed attributes. However, the decorator syntax allows for user-defined decorators as well. Specifically, for some function

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

224, this

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

is equivalent to

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

2. 17. 2 Pros

Elegantly specifies some transformation on a method; the transformation might eliminate some repetitive code, enforce invariants, etc

2. 17. 3 Cons

Decorators can perform arbitrary operations on a function’s arguments or return values, resulting in surprising implicit behavior. Additionally, decorators execute at object definition time. For module-level objects [classes, module functions, …] this happens at import time. Failures in decorator code are pretty much impossible to recover from

2. 17. 4 Decision

Use decorators judiciously when there is a clear advantage. Decorators should follow the same import and naming guidelines as functions. Decorator pydoc should clearly state that the function is a decorator. Write unit tests for decorators

Avoid external dependencies in the decorator itself [e. g. don’t rely on files, sockets, database connections, etc. ], since they might not be available when the decorator runs [at import time, perhaps from

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

225 or other tools]. A decorator that is called with valid parameters should [as much as possible] be guaranteed to succeed in all cases

Decorators are a special case of “top level code” - see main for more discussion

Never use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

220 unless forced to in order to integrate with an API defined in an existing library. Write a module level function instead

Use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

221 only when writing a named constructor, or a class-specific routine that modifies necessary global state such as a process-wide cache

2. 18 Threading

Do not rely on the atomicity of built-in types

While Python’s built-in data types such as dictionaries appear to have atomic operations, there are corner cases where they aren’t atomic [e. g. if

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

228 or

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

229 are implemented as Python methods] and their atomicity should not be relied upon. Neither should you rely on atomic variable assignment [since this in turn depends on dictionaries]

Use the Queue module’s

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

230 data type as the preferred way to communicate data between threads. Otherwise, use the threading module and its locking primitives. Prefer condition variables and

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

231 instead of using lower-level locks

2. 19 Power Features

Avoid these features

2. 19. 1 Definition

Python is an extremely flexible language and gives you many fancy features such as custom metaclasses, access to bytecode, on-the-fly compilation, dynamic inheritance, object reparenting, import hacks, reflection [e. g. some uses of

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

232], modification of system internals,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

233 methods implementing customized cleanup, etc

2. 19. 2 Pros

These are powerful language features. They can make your code more compact

2. 19. 3 Cons

It’s very tempting to use these “cool” features when they’re not absolutely necessary. It’s harder to read, understand, and debug code that’s using unusual features underneath. It doesn’t seem that way at first [to the original author], but when revisiting the code, it tends to be more difficult than code that is longer but is straightforward

2. 19. 4 Decision

Avoid these features in your code

Standard library modules and classes that internally use these features are okay to use [for example,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

234,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

235, and

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

236]

2. 20 Modern Python. from future imports

New language version semantic changes may be gated behind a special future import to enable them on a per-file basis within earlier runtimes

2. 20. 1 Definition

Being able to turn on some of the more modern features via

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

237 statements allows early use of features from expected future Python versions

2. 20. 2 Pros

This has proven to make runtime version upgrades smoother as changes can be made on a per-file basis while declaring compatibility and preventing regressions within those files. Modern code is more maintainable as it is less likely to accumulate technical debt that will be problematic during future runtime upgrades

2. 20. 3 Cons

Such code may not work on very old interpreter versions prior to the introduction of the needed future statement. The need for this is more common in projects supporting an extremely wide variety of environments

2. 20. 4 Decision

from __future__ imports

Use of

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

237 statements is encouraged. It allows a given source file to start using more modern Python syntax features today. Once you no longer need to run on a version where the features are hidden behind a

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

239 import, feel free to remove those lines

In code that may execute on versions as old as 3. 5 rather than >= 3. 7, import

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

For more information read the Python future statement definitions documentation

Please don’t remove these imports until you are confident the code is only ever used in a sufficiently modern environment. Even if you do not currently use the feature a specific future import enables in your code today, keeping it in place in the file prevents later modifications of the code from inadvertently depending on the older behavior

Use other

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

240 import statements as you see fit

2. 21 Type Annotated Code

You can annotate Python code with type hints according to PEP-484, and type-check the code at build time with a type checking tool like pytype

Type annotations can be in the source or in a stub pyi file. Whenever possible, annotations should be in the source. Use pyi files for third-party or extension modules

2. 21. 1 Definition

Type annotations [or “type hints”] are for function or method arguments and return values

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

You can also declare the type of a variable using similar PEP-526 syntax

2. 21. 2 Pros

Type annotations improve the readability and maintainability of your code. The type checker will convert many runtime errors to build-time errors, and reduce your ability to use Power Features

2. 21. 3 Cons

You will have to keep the type declarations up to date. You might see type errors that you think are valid code. Use of a type checker may reduce your ability to use Power Features

2. 21. 4 Decision

You are strongly encouraged to enable Python type analysis when updating code. When adding or modifying public APIs, include type annotations and enable checking via pytype in the build system. As static analysis is relatively new to Python, we acknowledge that undesired side-effects [such as wrongly inferred types] may prevent adoption by some projects. In those situations, authors are encouraged to add a comment with a TODO or link to a bug describing the issue[s] currently preventing type annotation adoption in the BUILD file or in the code itself as appropriate

3 Python Style Rules

3. 1 Semicolons

Do not terminate your lines with semicolons, and do not use semicolons to put two statements on the same line

3. 2 Line length

Maximum line length is 80 characters

Explicit exceptions to the 80 character limit

Long import statements
URLs, pathnames, or long flags in comments
Long string module level constants not containing whitespace that would be inconvenient to split across lines such as URLs or pathnames
- Pylint disable comments. [e. g.
```
def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam
```
  241]

Do not use backslash line continuation except for

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

242 statements requiring three or more context managers

Make use of Python’s implicit line joining inside parentheses, brackets and braces. If necessary, you can add an extra pair of parentheses around an expression

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

When a literal string won’t fit on a single line, use parentheses for implicit line joining

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

Within comments, put long URLs on their own line if necessary

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

It is permissible to use backslash continuation when defining a

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

242 statement with three or more context managers. For two context managers, use a nested

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

242 statement

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

Make note of the indentation of the elements in the line continuation examples above; see the indentation section for explanation

In all other cases where a line exceeds 80 characters, and the yapf auto-formatter does not help bring the line below the limit, the line is allowed to exceed this maximum. Authors are encouraged to manually break the line up per the notes above when it is sensible

3. 3 Parentheses

Use parentheses sparingly

It is fine, though not required, to use parentheses around tuples. Do not use them in return statements or conditional statements unless using parentheses for implied line continuation or to indicate a tuple

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

3. 4 Indentation

Indent your code blocks with 4 spaces

Never use tabs. Implied line continuation should align wrapped elements vertically [see line length examples], or use a hanging 4-space indent. Closing [round, square or curly] brackets can be placed at the end of the expression, or on separate lines, but then should be indented the same as the line with the corresponding opening bracket

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

3. 4. 1 Trailing commas in sequences of items?

Trailing commas in sequences of items are recommended only when the closing container token

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

245,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

246, or

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

247 does not appear on the same line as the final element. The presence of a trailing comma is also used as a hint to our Python code auto-formatter YAPF to direct it to auto-format the container of items to one item per line when the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

248 after the final element is present

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

3. 5 Blank Lines

Two blank lines between top-level definitions, be they function or class definitions. One blank line between method definitions and between the docstring of a

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

249 and the first method. No blank line following a

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

250 line. Use single blank lines as you judge appropriate within functions or methods

Blank lines need not be anchored to the definition. For example, related comments immediately preceding function, class, and method definitions can make sense. Consider if your comment might be more useful as part of the docstring

3. 6 Whitespace

Follow standard typographic rules for the use of spaces around punctuation

No whitespace inside parentheses, brackets or braces

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

No whitespace before a comma, semicolon, or colon. Do use whitespace after a comma, semicolon, or colon, except at the end of the line

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

No whitespace before the open paren/bracket that starts an argument list, indexing or slicing

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

No trailing whitespace

Surround binary operators with a single space on either side for assignment [

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

251], comparisons [

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

252], and Booleans [

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

253]. Use your better judgment for the insertion of spaces around arithmetic operators [

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

254,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

255,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

256,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

257,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

258,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

259,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

260,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

222]

Never use spaces around

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

251 when passing keyword arguments or defining a default parameter value, with one exception. when a type annotation is present, do use spaces around the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

251 for the default parameter value

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

Don’t use spaces to vertically align tokens on consecutive lines, since it becomes a maintenance burden [applies to

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

264,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

265,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

251, etc. ]

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

3. 7 Shebang Line

Most

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

267 files do not need to start with a

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

268 line. Start the main file of a program with

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

269 [to support virtualenvs] or

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

270 per PEP-394

This line is used by the kernel to find the Python interpreter, but is ignored by Python when importing modules. It is only necessary on a file intended to be executed directly

Be sure to use the right style for module, function, method docstrings and inline comments

3. 8. 1 Docstrings

Python uses docstrings to document code. A docstring is a string that is the first statement in a package, module, class or function. These strings can be extracted automatically through the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

271 member of the object and are used by

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

225. [Try running

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

225 on your module to see how it looks. ] Always use the three double-quote

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

274 format for docstrings [per PEP 257]. A docstring should be organized as a summary line [one physical line not exceeding 80 characters] terminated by a period, question mark, or exclamation point. When writing more [encouraged], this must be followed by a blank line, followed by the rest of the docstring starting at the same cursor position as the first quote of the first line. There are more formatting guidelines for docstrings below

3. 8. 2 Modules

Every file should contain license boilerplate. Choose the appropriate boilerplate for the license used by the project [for example, Apache 2. 0, BSD, LGPL, GPL]

Files should start with a docstring describing the contents and usage of the module

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

3. 8. 2. 1 Test modules

Module-level docstrings for test files are not required. They should be included only when there is additional information that can be provided

Examples include some specifics on how the test should be run, an explanation of an unusual setup pattern, dependency on the external environment, and so on

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

Docstrings that do not provide any new information should not be used

3. 8. 3 Functions and Methods

In this section, “function” means a method, function, generator, or property

A docstring is mandatory for every function that has one or more of the following properties

being part of the public API
nontrivial size
non-obvious logic

A docstring should give enough information to write a call to the function without reading the function’s code. The docstring should describe the function’s calling syntax and its semantics, but generally not its implementation details, unless those details are relevant to how the function is to be used. For example, a function that mutates one of its arguments as a side effect should note that in its docstring. Otherwise, subtle but important details of a function’s implementation that are not relevant to the caller are better expressed as comments alongside the code than within the function’s docstring

The docstring may be descriptive-style [

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

275] or imperative-style [

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

276], but the style should be consistent within a file. The docstring for a

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

91 data descriptor should use the same style as the docstring for an attribute or a function argument [

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

278, rather than

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

279]

A method that overrides a method from a base class may have a simple docstring sending the reader to its overridden method’s docstring, such as

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

280. The rationale is that there is no need to repeat in many places documentation that is already present in the base method’s docstring. However, if the overriding method’s behavior is substantially different from the overridden method, or details need to be provided [e. g. , documenting additional side effects], a docstring with at least those differences is required on the overriding method

Certain aspects of a function should be documented in special sections, listed below. Each section begins with a heading line, which ends with a colon. All sections other than the heading should maintain a hanging indent of two or four spaces [be consistent within a file]. These sections can be omitted in cases where the function’s name and signature are informative enough that it can be aptly described using a one-line docstring

Args. List each parameter by name. A description should follow the name, and be separated by a colon followed by either a space or newline. If the description is too long to fit on a single 80-character line, use a hanging indent of 2 or 4 spaces more than the parameter name [be consistent with the rest of the docstrings in the file]. The description should include required type[s] if the code does not contain a corresponding type annotation. If a function accepts

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

281 [variable length argument lists] and/or

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

282 [arbitrary keyword arguments], they should be listed as

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

281 and

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

282. Returns. [or Yields. for generators]Describe the type and semantics of the return value. If the function only returns None, this section is not required. It may also be omitted if the docstring starts with Returns or Yields [e. g.

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

285] and the opening sentence is sufficient to describe the return value. Do not imitate ‘NumPy style’ [example], which frequently documents a tuple return value as if it were multiple return values with individual names [never mentioning the tuple]. Instead, describe such a return value as. “Returns. A tuple [mat_a, mat_b], where mat_a is …, and …”. The auxiliary names in the docstring need not necessarily correspond to any internal names used in the function body [as those are not part of the API]. Raises. List all exceptions that are relevant to the interface followed by a description. Use a similar exception name + colon + space or newline and hanging indent style as described in Args. You should not document exceptions that get raised if the API specified in the docstring is violated [because this would paradoxically make behavior under violation of the API part of the API]

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

Similarly, this variation on

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

286 with a line break is also allowed

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

3. 8. 4 Classes

Classes should have a docstring below the class definition describing the class. If your class has public attributes, they should be documented here in an

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

287 section and follow the same formatting as a function’s

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

288 section

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

All class docstrings should start with a one-line summary that describes what the class instance represents. This implies that subclasses of

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

60 should also describe what the exception represents, and not the context in which it might occur. The class docstring should not repeat unnecessary information, such as that the class is a class

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

3. 8. 5 Block and Inline Comments

The final place to have comments is in tricky parts of the code. If you’re going to have to explain it at the next code review, you should comment it now. Complicated operations get a few lines of comments before the operations commence. Non-obvious ones get comments at the end of the line

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

To improve legibility, these comments should start at least 2 spaces away from the code with the comment character

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

265, followed by at least one space before the text of the comment itself

On the other hand, never describe the code. Assume the person reading the code knows Python [though not what you’re trying to do] better than you do

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

3. 8. 6 Punctuation, Spelling, and Grammar

Pay attention to punctuation, spelling, and grammar; it is easier to read well-written comments than badly written ones

Comments should be as readable as narrative text, with proper capitalization and punctuation. In many cases, complete sentences are more readable than sentence fragments. Shorter comments, such as comments at the end of a line of code, can sometimes be less formal, but you should be consistent with your style

Although it can be frustrating to have a code reviewer point out that you are using a comma when you should be using a semicolon, it is very important that source code maintain a high level of clarity and readability. Proper punctuation, spelling, and grammar help with that goal

3. 10 Strings

Use an f-string, the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

259 operator, or the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

292 method for formatting strings, even when the parameters are all strings. Use your best judgment to decide between string formatting options. A single join with

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

254 is okay but do not format with

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

254

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

Avoid using the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

254 and

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

296 operators to accumulate a string within a loop. In some conditions, accumulating a string with addition can lead to quadratic rather than linear running time. Although common accumulations of this sort may be optimized on CPython, that is an implementation detail. The conditions under which an optimization applies are not easy to predict and may change. Instead, add each substring to a list and

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

297 the list after the loop terminates, or write each substring to an

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

298 buffer. These techniques consistently have amortized-linear run time complexity

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

Be consistent with your choice of string quote character within a file. Pick

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

299 or

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

300 and stick with it. It is okay to use the other quote character on a string to avoid the need to backslash-escape quote characters within the string

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin

Prefer

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

274 for multi-line strings rather than

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

302. Projects may choose to use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

302 for all non-docstring multi-line strings if and only if they also use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

299 for regular strings. Docstrings must use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

274 regardless

Multi-line strings do not flow with the indentation of the rest of the program. If you need to avoid embedding extra space in the string, use either concatenated single-line strings or a multi-line string with

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

306 to remove the initial space on each line

dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin

dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin

dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin

dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin

dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin

3. 10. 1 Logging

For logging functions that expect a pattern-string [with %-placeholders] as their first argument. Always call them with a string literal [not an f-string. ] as their first argument with pattern-parameters as subsequent arguments. Some logging implementations collect the unexpanded pattern-string as a queryable field. It also prevents spending time rendering a message that no logger is configured to output

dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin

dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin

dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin

3. 10. 2 Error Messages

Error messages [such as. message strings on exceptions like

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

54, or messages shown to the user] should follow three guidelines

The message needs to precisely match the actual error condition
Interpolated pieces need to always be clearly identifiable as such
They should allow simple automated processing [e. g. grepping]

dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin

dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin

3. 11 Files, Sockets, and similar Stateful Resources

Explicitly close files and sockets when done with them. This rule naturally extends to closeable resources that internally use sockets, such as database connections, and also other resources that need to be closed down in a similar fashion. To name only a few examples, this also includes mmap mappings, h5py File objects, and matplotlib. pyplot figure windows

Leaving files, sockets or other such stateful objects open unnecessarily has many downsides

They may consume limited system resources, such as file descriptors. Code that deals with many such objects may exhaust those resources unnecessarily if they’re not returned to the system promptly after use
Holding files open may prevent other actions such as moving or deleting them, or unmounting a filesystem
Files and sockets that are shared throughout a program may inadvertently be read from or written to after logically being closed. If they are actually closed, attempts to read or write from them will raise exceptions, making the problem known sooner

Furthermore, while files and sockets [and some similarly behaving resources] are automatically closed when the object is destructed, coupling the lifetime of the object to the state of the resource is poor practice

There are no guarantees as to when the runtime will actually invoke the
```
def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam
```
233 method. Different Python implementations use different memory management techniques, such as delayed garbage collection, which may increase the object’s lifetime arbitrarily and indefinitely
Unexpected references to the file, e. g. in globals or exception tracebacks, may keep it around longer than intended

Relying on finalizers to do automatic cleanup that has observable side effects has been rediscovered over and over again to lead to major problems, across many decades and multiple languages [see e. g. this article for Java]

The preferred way to manage files and similar resources is using the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

242 statement

dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin

For file-like objects that do not support the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

242 statement, use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

311

dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin

In rare cases where context-based resource management is infeasible, code documentation must explain clearly how resource lifetime is managed

Use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

312 comments for code that is temporary, a short-term solution, or good-enough but not perfect

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

312 comment begins with the word

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

312 in all caps, and a parenthesized context identifier. Ideally a bug reference, sometimes a username. A bug reference like

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

315 is preferable, because bugs are tracked and have follow-up comments, whereas individuals move around and may lose context over time. The

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

312 is followed by an explanation of what there is to do

The purpose is to have a consistent

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

312 format that can be searched to find out how to get more details. A

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

312 is not a commitment that the person referenced will fix the problem. Thus when you create a

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

312 with a username, it is almost always your own username that is given

dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin

If your

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

312 is of the form “At a future date do something” make sure that you either include a very specific date [“Fix by November 2009”] or a very specific event [“Remove this code when all clients can handle XML responses. ”] that future code maintainers will comprehend

3. 13 Imports formatting

Imports should be on separate lines; there are exceptions for

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

321 and

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

322 imports

E. g

dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin

Imports are always put at the top of the file, just after any module comments and docstrings and before module globals and constants. Imports should be grouped from most generic to least generic

Python future import statements. For example
```
dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin
```
65
See above for more information about those
Python standard library imports. For example
third-party module or package imports. For example

Code repository sub-package imports. For example

dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin

Deprecated. application-specific imports that are part of the same top level sub-package as this file. For example
```
dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin
```
67
You may find older Google Python Style code doing this, but it is no longer required. New code is encouraged not to bother with this. Simply treat application-specific sub-package imports the same as other sub-package imports

Within each grouping, imports should be sorted lexicographically, ignoring case, according to each module’s full package path [the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

323 in

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

324]. Code may optionally place a blank line between import sections

dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin

3. 14 Statements

Generally only one statement per line

However, you may put the result of a test on the same line as the test only if the entire statement fits on one line. In particular, you can never do so with

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

63/

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

64 since the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

63 and

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

64 can’t both fit on the same line, and you can only do so with an

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

329 if there is no

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

330

dict = 'something awful'  # Bad Idea.. pylint: disable=redefined-builtin

3. 15 Getters and Setters

Getter and setter functions [also called accessors and mutators] should be used when they provide a meaningful role or behavior for getting or setting a variable’s value

In particular, they should be used when getting or setting the variable is complex or the cost is significant, either currently or in a reasonable future

If, for example, a pair of getters/setters simply read and write an internal attribute, the internal attribute should be made public instead. By comparison, if setting a variable means some state is invalidated or rebuilt, it should be a setter function. The function invocation hints that a potentially non-trivial operation is occurring. Alternatively, properties may be an option when simple logic is needed, or refactoring to no longer need getters and setters

Getters and setters should follow the Naming guidelines, such as

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

331 and

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

332

If the past behavior allowed access through a property, do not bind the new getter/setter functions to the property. Any code still attempting to access the variable by the old method should break visibly so they are made aware of the change in complexity

3. 16 Naming

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

333,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

334,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

335,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

336,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

337,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

338,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

339,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

340,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

341,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

342,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

343,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

344,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

345

Function names, variable names, and filenames should be descriptive; avoid abbreviation. In particular, do not use abbreviations that are ambiguous or unfamiliar to readers outside your project, and do not abbreviate by deleting letters within a word

Always use a

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

267 filename extension. Never use dashes

3. 16. 1 Names to Avoid

single character names, except for specifically allowed cases

counters or iterators [e. g.

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

347,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

348,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

349,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

350, et al. ]

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

351 as an exception identifier in

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

352 statements

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

353 as a file handle in

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

242 statements

private

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

355s with no constraints [e. g.

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

356,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

357,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

358]

Please be mindful not to abuse single-character naming. Generally speaking, descriptiveness should be proportional to the name’s scope of visibility. For example,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

347 might be a fine name for 5-line code block but within multiple nested scopes, it is likely too vague

dashes [

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

255] in any package/module name

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

361 names [reserved by Python]

offensive terms

names that needlessly include the type of the variable [for example.

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

362]

3. 16. 2 Naming Conventions

“Internal” means internal to a module, or protected or private within a class

Prepending a single underscore [

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

28] has some support for protecting module variables and functions [linters will flag protected member access]

Prepending a double underscore [
```
def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam
```
364 aka “dunder”] to an instance variable or method effectively makes the variable or method private to its class [using name mangling]; we discourage its use as it impacts readability and testability, and isn’t really private. Prefer a single underscore
Place related classes and top-level functions together in a module. Unlike Java, there is no need to limit yourself to one class per module

Use CapWords for class names, but lower_with_under. py for module names. Although there are some old modules named CapWords. py, this is now discouraged because it’s confusing when the module happens to be named after a class. [“wait – did I write

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

365 or

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

366?”]

Underscores may appear in unittest method names starting with

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

367 to separate logical components of the name, even if those components use CapWords. One possible pattern is

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

368; for example

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

369 is okay. There is no One Correct Way to name test methods

3. 16. 3 File Naming

Python filenames must have a

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

267 extension and must not contain dashes [

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

255]. This allows them to be imported and unittested. If you want an executable to be accessible without the extension, use a symbolic link or a simple bash wrapper containing

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

372

3. 16. 4 Guidelines derived from Guido’s Recommendations

TypePublicInternalPackages

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

373Modules

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

373

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

375Classes

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

376

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

377Exceptions

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

376Functions

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

379

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

380Global/Class Constants

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

381

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

382Global/Class Variables

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

373

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

375Instance Variables

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

373

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

375 [protected]Method Names

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

379

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

380 [protected]Function/Method Parameters

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

373Local Variables

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

373

3. 16. 5 Mathematical Notation

For mathematically heavy code, short variable names that would otherwise violate the style guide are preferred when they match established notation in a reference paper or algorithm. When doing so, reference the source of all naming conventions in a comment or docstring or, if the source is not accessible, clearly document the naming conventions. Prefer PEP8-compliant

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

391 for public APIs, which are much more likely to be encountered out of context

3. 17 Main

In Python,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

225 as well as unit tests require modules to be importable. If a file is meant to be used as an executable, its main functionality should be in a

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

393 function, and your code should always check

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

394 before executing your main program, so that it is not executed when the module is imported

When using absl, use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

395

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

Otherwise, use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

All code at the top level will be executed when the module is imported. Be careful not to call functions, create objects, or perform other operations that should not be executed when the file is being

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

225ed

3. 18 Function length

Prefer small and focused functions

We recognize that long functions are sometimes appropriate, so no hard limit is placed on function length. If a function exceeds about 40 lines, think about whether it can be broken up without harming the structure of the program

Even if your long function works perfectly now, someone modifying it in a few months may add new behavior. This could result in bugs that are hard to find. Keeping your functions short and simple makes it easier for other people to read and modify your code

You could find long and complicated functions when working with some code. Do not be intimidated by modifying existing code. if working with such a function proves to be difficult, you find that errors are hard to debug, or you want to use a piece of it in several different contexts, consider breaking up the function into smaller and more manageable pieces

3. 19 Type Annotations

3. 19. 1 General Rules

Familiarize yourself with PEP-484

In methods, only annotate

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

73, or

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

74 if it is necessary for proper type information. e. g. ,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

Similarly, don’t feel compelled to annotate the return value of

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

399 [where

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

98 is the only valid option]

If any other variable or a returned type should not be expressed, use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

401

You are not required to annotate all the functions in a module
- At least annotate your public APIs
- Use judgment to get to a good balance between safety and clarity on the one hand, and flexibility on the other
- Annotate code that is prone to type-related errors [previous bugs or complexity]
- Annotate code that is hard to understand
- Annotate code as it becomes stable from a types perspective. In many cases, you can annotate all the functions in mature code without losing too much flexibility

3. 19. 2 Line Breaking

Try to follow the existing indentation rules

After annotating, many function signatures will become “one parameter per line”. To ensure the return type is also given its own line, a comma can be placed after the last parameter

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

Always prefer breaking between variables, and not, for example, between variable names and type annotations. However, if everything fits on the same line, go for it

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

If the combination of the function name, the last parameter, and the return type is too long, indent by 4 in a new line. When using line breaks, prefer putting each parameter and the return type on their own lines and aligning the closing parenthesis with the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

250

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

Optionally, the return type may be put on the same line as the last parameter

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

18 allows you to move the closing parenthesis to a new line and align with the opening one, but this is less readable

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

As in the examples above, prefer not to break types. However, sometimes they are too long to be on a single line [try to keep sub-types unbroken]

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

If a single name and type is too long, consider using an alias for the type. The last resort is to break after the colon and indent by 4

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

3. 19. 3 Forward Declarations

If you need to use a class name [from the same module] that is not yet defined – for example, if you need the class name inside the declaration of that class, or if you use a class that is defined later in the code – either use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

404 or use a string for the class name

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

3. 19. 4 Default Values

As per PEP-008, use spaces around the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

251 only for arguments that have both a type annotation and a default value

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

3. 19. 5 NoneType

In the Python type system,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

406 is a “first class” type, and for typing purposes,

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

98 is an alias for

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

406. If an argument can be

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

98, it has to be declared. You can use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

410, but if there is only one other type, use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

411

Use explicit

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

411 instead of implicit

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

411. Earlier versions of PEP 484 allowed

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

414 to be interpreted as

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

415, but that is no longer the preferred behavior

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

3. 19. 6 Type Aliases

You can declare aliases of complex types. The name of an alias should be CapWorded. If the alias is used only in this module, it should be _Private

For example, if the name of the module together with the name of the type is too long

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

Other examples are complex nested types and multiple return variables from a function [as a tuple]

3. 19. 7 Ignoring Types

You can disable type checking on a line with the special comment

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

416

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

417 has a disable option for specific errors [similar to lint]

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

3. 19. 8 Typing Variables

Annotated AssignmentsIf an internal variable has a type that is hard or impossible to infer, specify its type with an annotated assignment - use a colon and type between the variable name and value [the same as is done with function arguments that have a default value]

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

Type CommentsThough you may see them remaining in the codebase [they were necessary before Python 3. 6], do not add any more uses of a

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

418 comment on the end of the line

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

3. 19. 9 Tuples vs Lists

Typed lists can only contain objects of a single type. Typed tuples can either have a single repeated type or a set number of elements with different types. The latter is commonly used as the return type from a function

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

3. 19. 10 TypeVars

The Python type system has generics. The factory function

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

355 is a common way to use them

Example

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

A TypeVar can be constrained

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

A common predefined type variable in the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

321 module is

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

421. Use it for multiple annotations that can be

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

422 or

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

423 and must all be the same type

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

A TypeVar must have a descriptive name, unless it meets all of the following criteria

not externally visible
not constrained

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

3. 19. 11 String types

Do not use
def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam
424 in new code. It’s only for Python 2/3 compatibility

Use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

423 for string/text data. For code that deals with binary data, use

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

422

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

If all the string types of a function are always the same, for example if the return type is the same as the argument type in the code above, use AnyStr

3. 19. 12 Imports For Typing

For symbols from the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

321 and

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

322 modules used to support static analysis and type checking, always import the symbol itself. This keeps common annotations more concise and matches typing practices used around the world. You are explicitly allowed to import multiple specific classes on one line from the

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

321 and

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

322 modules. Ex

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

Given that this way of importing adds items to the local namespace, names in

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

321 or

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

322 should be treated similarly to keywords, and not be defined in your Python code, typed or not. If there is a collision between a type and an existing name in a module, import it using

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

433

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

Prefer to use built-in types as annotations where available. Python supports type annotations using parametric container types via PEP-585, introduced in Python 3. 9

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

NOTE. Users of Apache Beam should continue to import parametric containers from

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

321

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

3. 19. 13 Conditional Imports

Use conditional imports only in exceptional cases where the additional imports needed for type checking must be avoided at runtime. This pattern is discouraged; alternatives such as refactoring the code to allow top level imports should be preferred

Imports that are needed only for type annotations can be placed within an

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

435 block

Conditionally imported types need to be referenced as strings, to be forward compatible with Python 3. 6 where the annotation expressions are actually evaluated
Only entities that are used solely for typing should be defined here; this includes aliases. Otherwise it will be a runtime error, as the module will not be imported at runtime
The block should be right after all the normal imports
There should be no empty lines in the typing imports list

Sort this list as if it were a regular imports list

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

3. 19. 14 Circular Dependencies

Circular dependencies that are caused by typing are code smells. Such code is a good candidate for refactoring. Although technically it is possible to keep circular dependencies, various build systems will not let you do so because each module has to depend on the other

Replace modules that create circular dependency imports with

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

401. Set an alias with a meaningful name, and use the real type name from this module [any attribute of Any is Any]. Alias definitions should be separated from the last import by one line

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

3. 19. 15 Generics

When annotating, prefer to specify type parameters for generic types; otherwise, the generics’ parameters will be assumed to be

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

401

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

If the best type parameter for a generic is

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

401, make it explicit, but remember that in many cases

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

355 might be more appropriate

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

def viking_cafe_order[spam: str, beans: str, eggs: Optional[str] = None] -> str:
    del beans, eggs  # Unused by vikings.
    return spam + spam + spam

4 Parting Words

BE CONSISTENT

If you’re editing code, take a few minutes to look at the code around you and determine its style. If they use spaces around all their arithmetic operators, you should too. If their comments have little boxes of hash marks around them, make your comments have little boxes of hash marks around them too

The point of having style guidelines is to have a common vocabulary of coding so people can concentrate on what you’re saying rather than on how you’re saying it. We present global style rules here so people know the vocabulary, but local style is also important. If code you add to a file looks drastically different from the existing code around it, it throws readers out of their rhythm when they go to read it. Avoid this