How to Check if String Contains Substring in Python

In Python, you can check if a string contains a substring using the in keyword, like substring in string. This expression returns True if the substring is found and False otherwise.

Syntax

The basic syntax to check if a substring exists in a string is:

• substring in string: Returns True if substring is found inside string.
• substring not in string: Returns True if substring is NOT found inside string.

python

substring in string
substring not in string

Example

This example shows how to check if the word 'cat' is inside a sentence string.

python

sentence = "The cat is sleeping on the sofa."
if "cat" in sentence:
    print("Found 'cat' in the sentence.")
else:
    print("'cat' not found in the sentence.")

Output

Found 'cat' in the sentence.

Common Pitfalls

One common mistake is using the wrong method like string.contains(substring), which does not exist in Python. Another is confusing in with methods like find() or index() which behave differently.

Also, remember that the in check is case-sensitive, so 'Cat' is different from 'cat'.

python

text = "Hello World"
# Wrong way (will cause error):
# if text.contains("World"):
#     print("Found")

# Right way:
if "World" in text:
    print("Found")

# Case sensitivity example:
if "world" in text:
    print("Found lowercase world")
else:
    print("Did not find lowercase world")

Output

Found
Did not find lowercase world

Quick Reference

Use this quick guide to remember how to check substring presence:

Operation	Description	Example	Result
Check substring	Returns True if substring is in string	"cat" in "concatenate"	True
Check substring absence	Returns True if substring is NOT in string	"dog" not in "concatenate"	True
Case sensitive	Check is case sensitive	"Cat" in "cat"	False

Key Takeaways

Use the in keyword to check if a substring exists in a string.

The in check returns a boolean: True if found, False if not.

Remember that substring checks are case-sensitive in Python.

Avoid using non-existent methods like contains() in Python strings.

Use not in to check if a substring is absent.