How do you handle an exception in a for loop in python?

Exceptions in Python

Python has many built-in exceptions that are raised when your program encounters an error [something in the program goes wrong].

When these exceptions occur, the Python interpreter stops the current process and passes it to the calling process until it is handled. If not handled, the program will crash.

For example, let us consider a program where we have a function A that calls function B, which in turn calls function C. If an exception occurs in function C but is not handled in C, the exception passes to B and then to A.

If never handled, an error message is displayed and our program comes to a sudden unexpected halt.

Catching Exceptions in Python

In Python, exceptions can be handled using a try statement.

The critical operation which can raise an exception is placed inside the try clause. The code that handles the exceptions is written in the except clause.

We can thus choose what operations to perform once we have caught the exception. Here is a simple example.

# import module sys to get the type of exception
import sys

randomList = ['a', 0, 2]

for entry in randomList:
    try:
        print["The entry is", entry]
        r = 1/int[entry]
        break
    except:
        print["Oops!", sys.exc_info[][0], "occurred."]
        print["Next entry."]
        print[]
print["The reciprocal of", entry, "is", r]

Output

The entry is a
Oops!  occurred.
Next entry.

The entry is 0
Oops!  occured.
Next entry.

The entry is 2
The reciprocal of 2 is 0.5

In this program, we loop through the values of the randomList list. As previously mentioned, the portion that can cause an exception is placed inside the try block.

If no exception occurs, the except block is skipped and normal flow continues[for last value]. But if any exception occurs, it is caught by the except block [first and second values].

Here, we print the name of the exception using the exc_info[] function inside sys module. We can see that a causes ValueError and 0 causes ZeroDivisionError.

Since every exception in Python inherits from the base Exception class, we can also perform the above task in the following way:

# import module sys to get the type of exception
import sys

randomList = ['a', 0, 2]

for entry in randomList:
    try:
        print["The entry is", entry]
        r = 1/int[entry]
        break
    except Exception as e:
        print["Oops!", e.__class__, "occurred."]
        print["Next entry."]
        print[]
print["The reciprocal of", entry, "is", r]

This program has the same output as the above program.

Catching Specific Exceptions in Python

In the above example, we did not mention any specific exception in the except clause.

This is not a good programming practice as it will catch all exceptions and handle every case in the same way. We can specify which exceptions an except clause should catch.

A try clause can have any number of except clauses to handle different exceptions, however, only one will be executed in case an exception occurs.

We can use a tuple of values to specify multiple exceptions in an except clause. Here is an example pseudo code.

try:
   # do something
   pass

except ValueError:
   # handle ValueError exception
   pass

except [TypeError, ZeroDivisionError]:
   # handle multiple exceptions
   # TypeError and ZeroDivisionError
   pass

except:
   # handle all other exceptions
   pass

Raising Exceptions in Python

In Python programming, exceptions are raised when errors occur at runtime. We can also manually raise exceptions using the raise keyword.

We can optionally pass values to the exception to clarify why that exception was raised.

>>> raise KeyboardInterrupt
Traceback [most recent call last]:
...
KeyboardInterrupt

>>> raise MemoryError["This is an argument"]
Traceback [most recent call last]:
...
MemoryError: This is an argument

>>> try:
...     a = int[input["Enter a positive integer: "]]
...     if a