Python String index() Method

The index() method returns the index of the first occurence of a substring in the given string. It is same as the find() method except that if a substring is not found, then it raises an exception.

Syntax:

str.index(substr, start, end)

Parameters:

1. substr: (Required) The substring whose index has to be found.
2. start: (Optional) The starting index position from where the searching should start in the string. Default is 0.
3. end: (Optional) The ending index position untill the searching should happen. Default is end of the string.

Return Value:

An integer value indicating an index of the specified substring.

The following examples demonstrates index() method.

greet='Hello World!'
print('Index of H: ', greet.index('H'))
print('Index of e: ', greet.index('e'))
print('Index of l: ', greet.index('l'))
print('Index of World: ', greet.index('World'))

Index of H: 0
Index of e: 1
Index of l: 2
Index of World: 6

The index() method returns an index of the first occurance only.

greet='Hello World'
print('Index of l: ', greet.index('l'))
print('Index of o: ', greet.index('o'))

Index of l: 2
Index of o: 4

The index() method performs case-sensitive search. It throws ValueError if a substring not found.

greet='Hello World'
print(greet.index('h')) # throws error: substring not found 
print(greet.index('hello')) # throws error

ValueError: substring not found
ValueError: substring not found

If a given substring could not be found then it will throw an error.

mystr='TutorialsTeacher is a free online learning website'
print(mystr.index('python'))

Traceback (most recent call last)
<ipython-input-3-a72aac0824ca> in <module>
      1 str='TutorialsTeacher is a free online learning website'
----> 2 print(str.index('python'))
ValueError: substring not found

Use start and end parameter to limit the search of a substring between the specified starting and ending index.

mystr='tutorialsteacher is a free tutorials website'
print(mystr.index('tutorials',10)) # search starts from 10th index
print(mystr.index('tutorials',1, 26)) # throws error; searches between 1st and 26th index

27
ValueError: substring not found