Python String index() Method – Find Position of a Substring

Python String index() Method

The index() method in Python is used to find the position of the first occurrence of a substring inside a string. If the substring is not found, it raises a ValueError.

Syntax

string.index(substring, start, end)

Parameters:

  • substring – The string to search for.
  • start – (Optional) The position to start searching from. Default is 0.
  • end – (Optional) The position to stop searching. Default is end of the string.

Returns:

  • The index (integer) of the first match of the substring.

Example 1: Basic Usage

text = "Python is fun"
print(text.index("fun"))
10

Explanation: The word "fun" starts at index 10 in the string.

Example 2: With Start and End Parameters

text = "banana"
print(text.index("a", 2))
3

Explanation: It starts searching from index 2, so it finds the second "a" at position 3.

Difference Between index() and find()

  • index() raises a ValueError if the substring is not found.
  • find() returns -1 instead of raising an error.

Common Error

text = "hello"
print(text.index("z"))  # Raises ValueError
ValueError: substring not found

Use Case: Validate Substring Presence

text = "apple pie"
if "pie" in text:
    print("Found at index:", text.index("pie"))
Found at index: 6

Summary

  • string.index() returns the index of the first occurrence of a substring.
  • It raises a ValueError if not found.
  • Use optional start and end to limit the search range.

Practice Problem

Write a program to find the position of the word "world" in the string "hello world" using index().

text = "hello world"
print("Index of 'world':", text.index("world"))

Expected Output:

6

Next Steps

Try using index() with different substrings and edge cases like empty strings or overlapping matches.

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