How do i check if a string contains in python?

In this guide, we'll take a look at how to check if a string contains a substring in Python. As usual, each approach we'll cover has different pros and cons.

The in Operator

The easiest way to check if a Python string contains a substring is to use the in operator.

The in operator is used to check data structures for membership in Python. It returns a Boolean (either True or False). To check if a string contains a substring in Python using the in operator, we simply invoke it on the superstring:

fullstring = "StackAbuse"
substring = "tack"

if substring in fullstring:
    print("Found!")
else:
    print("Not found!")

This operator is shorthand for calling an object's __contains__ method, and also works well for checking if an item exists in a list. It's worth noting that it's not null-safe, so if our fullstring was pointing to None, an exception would be thrown:

TypeError: argument of type 'NoneType' is not iterable

To avoid this, you'll first want to check whether it points to None or not:

fullstring = None
substring = "tack"

if fullstring != None and substring in fullstring:
    print("Found!")
else:
    print("Not found!")

The String.index() Method

The String type in Python has a method called index() that can be used to find the starting index of the first occurrence of a substring in a string.

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If the substring is not found, a ValueError exception is thrown, which can be handled with a try-except-else block:

fullstring = "StackAbuse"
substring = "tack"

try:
    fullstring.index(substring)
except ValueError:
    print("Not found!")
else:
    print("Found!")

This method is useful if you need to know the position of the substring, as opposed to just its existence within the full string.

The String.find() Method

The String type has another method called find which is more convenient to use than index(), because we don't need to worry about handling any exceptions.

If find() doesn't find a match, it returns -1, otherwise it returns the left-most index of the substring in the larger string.

fullstring = "StackAbuse"
substring = "tack"

if fullstring.find(substring) != -1:
    print("Found!")
else:
    print("Not found!")

If you'd prefer to avoid the need to catch errors, then this method should be favored over index().

Regular Expressions (RegEx)

Regular expressions provide a more flexible (albeit more complex) way to check strings for pattern matching. Python is shipped with a built-in module for regular expressions, called re. The re module contains a function called search(), which we can use to match a substring pattern:

from re import search

fullstring = "StackAbuse"
substring = "tack"

if search(substring, fullstring):
    print "Found!"
else:
    print "Not found!"

This method is best if you are needing a more complex matching function, like case insensitive matching. Otherwise the complication and slower speed of regex should be avoided for simple substring matching use-cases.

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One of the most common operations that programmers use on strings is to check whether a string contains some other string.

If you are coming to Python from Java, for instance, you might have used the contains method to check if some substring exists in another string.

In Python, there are two ways to achieve this.

First: Using the in operator

The easiest way is via Python’s in operator.

Let’s take a look at this example.

>>> str = "Messi is the best soccer player"
>>> "soccer" in str
True
>>> "football" in str
False

As you can see, the in operator returns True when the substring exists in the string.

Otherwise, it returns false.

This method is very straightforward, clean, readable, and idiomatic.

Second: Using the find method

Another method you can use is the string’s find method.

Unlike the in operator which is evaluated to a boolean value, the find method returns an integer.

This integer is essentially the index of the beginning of the substring if the substring exists, otherwise -1 is returned.

Let’s see the find method in action.

>>> str = "Messi is the best soccer player"
>>> str.find("soccer")
18
>>> str.find("Ronaldo")
-1
>>> str.find("Messi")
0

One cool thing about this method is you can optionally specify a start index and an end index to limit your search within.

For example

>>> str = "Messi is the best soccer player"
>>> str.find("soccer", 5, 25)
18
>>> str.find("Messi", 5, 25)
-1

Notice how a -1 was returned for “Messi” because you are limiting your search to the string between indices 5 and 25 only.

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How do i check if a string contains in python?

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Some Advanced Stuff

Assume for a second that Python has no built-in functions or methods that would check if a string contains another string.

How would you write a function to do that?

Well, an easy way is to brute force by checking if the substring exists starting from every possible position in the original string.

For larger strings, this process can be really slow.

There are better algorithms for string searching.

I highly recommend this article from TopCoder if you want to learn more and dive deeper into string searching algorithms.

For more coverage of other string searching algorithms not covered in the previous article, this wikipedia page is great.

If you go through the previous articles and study them, your next question would be “well what algorithm does Python actually use?”

These kinds of questions almost always require digging into the source code.

But you are in luck because Python’s implementation is open source.

Alright, let’s dig into the code.

How do i check if a string contains in python?

Perfect, I am happy the developers commented their code 🙂

It is very clear now that the find method uses a mix of boyer-moore and horspool algorithms.

Conclusion

You can use the in operator or the string’s find method to check if a string contains another string.

The in operator returns True if the substring exists in the string. Otherwise, it returns False.

The find method returns the index of the beginning of the substring if found, otherwise -1 is returned.

Python’s implementation (CPython) uses a mix of boyer-moore and horspool for string searching.

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How do I check if a string contains something?

The includes() method returns true if a string contains a specified string. Otherwise it returns false . The includes() method is case sensitive.

Is there a Contains function in Python?

contains() function is used to test if pattern or regex is contained within a string of a Series or Index. The function returns boolean Series or Index based on whether a given pattern or regex is contained within a string of a Series or Index.

How do you check if a string contains a particular character in Python?

Using in operator The Pythonic, fast way to check for the specific character in a string uses the in operator. It returns True if the character is found in the string and False otherwise. ch = '. '

What is __ contains __ in Python?

Python string __contains__() is an instance method and returns boolean value True or False depending on whether the string object contains the specified string object or not. Note that the Python string contains() method is case sensitive.