How to check if a substring is contained in another string in Python?

In Python, you can check whether a substring exists within another string using the in operator, or string methods like find(), index(), and __contains__().

A string in Python is a sequence of characters enclosed in quotes. You can use either single quotes '...' or double quotes "..." ?

text1 = "Hello"    # double quotes
text2 = 'Python'   # single quotes
print(text1)
print(text2)

Hello
Python

A substring is simply a part of a string. For example ?

text = "Python"
substring = "tho"
print(f"'{substring}' is a substring of '{text}'")

'tho' is a substring of 'Python'

Using the in Operator

The in operator is the most common and readable way to check if a substring exists. It returns True if found, False otherwise ?

text = "Python is fun"
result = "Python" in text
print(result)

# Check for non-existing substring
result2 = "Java" in text
print(result2)

True
False

Using the find() Method

The find() method returns the starting index of the first occurrence of the substring. If not found, it returns -1 ?

text = "Python is fun"
index = text.find("fun")
print(f"'fun' found at index: {index}")

# Check for non-existing substring
index2 = text.find("Java")
print(f"'Java' found at index: {index2}")

'fun' found at index: 10
'Java' found at index: -1

Using the index() Method

The index() method works like find() but raises a ValueError if the substring is not found ?

text = "Python is fun"
position = text.index("is")
print(f"'is' found at position: {position}")

'is' found at position: 7

Handling ValueError with index()

Use try-except to handle the exception when substring is not found ?

text = "Python is fun"
try:
    position = text.index("Java")
    print(f"Found at position: {position}")
except ValueError:
    print("Substring not found.")

Substring not found.

Using __contains__() Method

The __contains__() method is what Python calls internally when you use the in operator ?

text = "Python is fun"
result = text.__contains__("fun")
print(result)

True

Using Regular Expressions

For pattern matching and complex searches, use the re module ?

import re

text = "Python is fun"
if re.search("fun", text):
    print("Found")
else:
    print("Not found")

# Pattern matching example
if re.search(r"\bfun\b", text):  # Word boundary
    print("'fun' found as complete word")

Found
'fun' found as complete word

Comparison

Conclusion

Use the in operator for simple substring checks. Use find() when you need the position and want to avoid exceptions. Use re.search() for pattern-based searches.