Python String index() Method

The index() method in Python is used to find the position of a specified substring within a given string. It is similar to the find() method but raises a ValueError if the substring is not found, while find() returns -1. This can be helpful when we want to ensure that the substring exists in the string before proceeding. Let's see a simple use of index():

s = "Python programming"
p = s.index("prog")
print(p)

7

Explanation: index() method searches for the substring "prog" within "Python programming" and returns starting index 7.

Syntax of index()

s.index(substring, start=0, end=len(s))

Parameters:

• substring: The substring to locate within the string s.
• start (optional): The starting index for the search. Defaults to 0 if not provided.
• end (optional): The ending index for the search. If not provided, it defaults to the length of the string.

Return Type:

• Returns the lowest index of the substring if found in the given string.
• Raises a ValueError if the substring is not found in the specified range.

Example of index() Method

Let's understand the use of index() with the help of some examples.

Example 1: Using index() with only substring argument

This code demonstrates how to find the index of a substring within a string using the index() method.

s = "Python programming is powerful"
p = s.index("programming")
print(p)

7

Explanation:

• Here, we only provide the substring argument "programming".
• index() searches from the start of the string and finds "programming" at index 7, which it returns.

Example 2: Using index() with substring, start and end arguments

This code demonstrates how to find the index of a substring within a specific range of a string using the index() method with optional start and end parameters.

s = "Python programming is fun"
p = s.index("is", 10, 25)
print(p)

19

Explanation:

• We specify start=10 and end=25, so index() searches between these positions.
• It finds "is" at index 18 within this range and returns 18.

Related Articles:

• Python String find() Method
• Python String rindex() Method
• Python String rfind() Method