Understanding Python String index()

Python String index() method is used to return the index of a substring if it is found inside a string.

• In case if the substring is not there inside the string then this method raises an exception.
• This method basically returns the index of the first occurrence of the value we are searching in a string.
• In the case of non-existence of substring inside a string ValueError Exception is raised.

Below we have a basic syntax of String index() in Python:

string.index(value, start, end)

Python String **index()**: Parameters

Given below is a description of Parameters of index() as there are three parameters:

• value

This is a required parameter and is used to specify the value which is to be searched.

• start

This is an optional parameter and is used to specify where the search starts; its default value is 0.

• end

This is an optional parameter used to specify where to end the search. The default value is to the end of the string.

Python String **index()**: Returned Values

The values returned by this method are as follows:

• This method returns the first position of the substring found.
• If substring not found then ValueError exception will be raised.

Below we have an example to show the working of String index() function:

ab = 'We can do whatever we want'
sample = ab.index('w', 5, 20)
print(sample)

The output of the above is given below:

10

Both these methods are same but there is a major difference in case if the substring does not exist in the string:

• If we use the python find() method then it will return -1.
• On the other hand, if we are using the python index() then it will raise ValueError Exception.

Below we have a case where we are looking for the index of a value that does not exist inside the string. Let us see the code snippet with its output:

abc = 'We can do whatever we want'
gfh= abc.index('z')
print(gfh)

Given below is the output

Traceback (most recent call last): File "<pyshell#4>", line 1, in <module> gfh= abc.index('z') ValueError: substring not found

Below we have an example where we have not specified the value of start and end parameters of index() method. The code snippet is given below:

ab = 'Expecto Patronum'
data = ab.index('num')
print(data)

Output:

13

In this tutorial, we learned index() method of Strings in Python which is used to return the index of the substring if found. We saw its parameters and a few basic examples; we also differentiate it with find() method of strings further followed by a live example.