Python String index()

Summary: in this tutorial, you’ll learn how to use the Python string index() method to get the index of a substring in a string.

Introduction to Python String index() method #

The string index() method returns the lowest index of a substring in a string.

The following shows the syntax of the index() method:

str.index(sub[, start[, end]])Code language: CSS (css)

The index() method has three parameters:

• sub is the substring to search for in the str.
• start and end parameters are interpreted as in the slice notation str[start:end]. The slice specifies where to look for the substring sub. Both start and end parameters are optional.

If the str doesn’t contain the substring sub within the slice str[start:end], the index() method will raise a ValueError.

If you don’t want to receive the ValueError when the sub is not found, you can use the string find() method instead. The find() method returns -1 instead of raising the ValueError.

Note that if you just want to check if a substring is in a string, you can use the in operator.

Python string index() method examples #

Let’s take some examples of using the index() method.

1) Using the Python string index() method to find the position of a substring in a string #

The following example shows how to use the index() method to find the substring 'Python' in the string 'Python will, Python will rock you.':

s = 'Python will, Python will rock you.'
position = s.index('Python')

print(position)Code language: PHP (php)

Output:

0

Since the string has two substrings 'Python', the index() method returns the lowest index where the substring found in the string.

2) Using the Python string index() method to find the position of a substring in a string within a slice #

The following example uses the index() method to find the substring 'Python' in the string 'Python will, Python will rock you.' within the slice str[1:]:

s = 'Python will, Python will rock you.'
position = s.index('Python', 1)

print(position)Code language: PHP (php)

Output:

13

In this example, the index() method returns the position of the second occurrence of the substring in the string.

3) The substring doesn’t exist in the string example #

The following example raises a ValueError because the substring 'Java' doesn’t exist in the string 'Python will, Python will rock you.':

s = 'Python will, Python will rock you.'
position = s.index('Java')

print(position)Code language: PHP (php)

Output:

ValueError: substring not foundCode language: HTTP (http)

Summary #

• Use the index() to get the lowest index of a substring within a string.
• The index() method raises a ValueError if the string doesn’t contain the substring.