How to Find Substring in String in Python: Simple Guide

In Python, you can find a substring in a string using the in keyword to check if it exists, or use str.find() and str.index() methods to get the position of the substring. find() returns -1 if not found, while index() raises an error.

Syntax

Here are the common ways to find a substring in a string:

• substring in string: Returns True if substring exists, else False.
• string.find(substring): Returns the lowest index of substring or -1 if not found.
• string.index(substring): Returns the lowest index of substring or raises ValueError if not found.

python

text = "hello world"

# Check if substring exists
exists = "world" in text

# Find position of substring
pos_find = text.find("world")
pos_index = text.index("world")

Example

This example shows how to check if a substring exists and find its position using in, find(), and index().

python

text = "hello world"
substring = "world"

# Check existence
if substring in text:
    print(f"'{substring}' found in text")
else:
    print(f"'{substring}' not found in text")

# Find position with find()
pos = text.find(substring)
print(f"Position using find(): {pos}")

# Find position with index()
pos2 = text.index(substring)
print(f"Position using index(): {pos2}")

Output

'world' found in text
Position using find(): 6
Position using index(): 6

Common Pitfalls

Common mistakes include:

• Using index() without handling ValueError if substring is missing.
• Confusing find() returning -1 with a valid index.
• Assuming in returns the position instead of a boolean.

python

text = "hello world"
substring = "python"

# Wrong: Using index() without try-except
try:
    pos = text.index(substring)
except ValueError:
    pos = -1
print(f"Position with index(): {pos}")

# Correct: Using find() which returns -1 if not found
pos_find = text.find(substring)
print(f"Position with find(): {pos_find}")

Output

Position with index(): -1
Position with find(): -1

Quick Reference

Method	Description	Return Value if Found	Return Value if Not Found
in	Check if substring exists	True	False
str.find()	Find lowest index of substring	Index (int)	-1
str.index()	Find lowest index of substring	Index (int)	Raises ValueError

Key Takeaways

Use in to quickly check if a substring exists in a string.

Use str.find() to get the position or -1 if substring is missing.

Use str.index() when you want an error if substring is not found.

Always handle ValueError when using index() to avoid crashes.

Remember in returns a boolean, not the position.