Python List index() Method Explained with Examples

What is the index() Function?

The method index() returns the lowest index in the list where the element searched for appears.

Let's try it out:

list_numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
element = 3
list_numbers.index(element)

2

If any element which is not present is searched, it returns a ValueError.

list_numbers = [4, 5, 6, 7, 8, 9, 10]
element = 3 # Not in the list
list_numbers.index(element) # Throws a ValueError

---------------------------------------------------------------------------

ValueError                                Traceback (most recent call last)

<ipython-input-35-6a006c988b06> in <module>()
      1 list_numbers = [4, 5, 6, 7, 8, 9, 10]
      2 element = 3 # Not in the list
----> 3 list_numbers.index(element) # Throws a ValueError


ValueError: 3 is not in list

Working with Strings

Let's try it with string elements:

list_numbers = [1, 'two', 3, 4, 5, 6, 7, 8, 9, 10]
element = 'two'
list_numbers.index(element)

1

Note: When searching strings, index() is case-sensitive. For example, 'Hello'.index('h') will raise a ValueError.

Finding the First Occurrence

Imagine you have more than one instance of an element, then index() will give you the first position where the element appears.

list_numbers = [3, 1, 2, 3, 3, 4, 5, 6, 3, 7, 8, 9, 10]
element = 3
list_numbers.index(element)

0

The position returned is 0, because 3 first appears in the first position or the 0th index in Python.

Here is what's happening internally: The index is going through all values starting from the 1st position (0th index), looking for the element you are searching for. When it finds the value - it returns the position and exits the system. However, this is not too efficient when going through a large list, and you need to get the position of something towards the end of the list.

Using Start and Stop Parameters

list_numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
element = 7
list_numbers.index(element, 5, 8)

6

So the syntax is: list_name.index(element, start, stop).

Here the start and stop values are optional. In fact, only use it when entirely sure about the range; otherwise, you will get a ValueError, as shown below.

list_numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
element = 7
list_numbers.index(element, 1, 5)

---------------------------------------------------------------------------

ValueError                                Traceback (most recent call last)

<ipython-input-39-b96f4b5e51d5> in <module>()
      1 list_numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
      2 element = 7
----> 3 list_numbers.index(element, 1, 5)


ValueError: 7 is not in list

Finding All Occurrences

Since the index() only returns the first match to an object, you can use list comprehension, or generator expression if you need the positions of more matches in the list. Here is how:

list_numbers = [3, 1, 2, 3, 3, 4, 5, 6, 3, 7, 8, 9, 10]
[i for i, n in enumerate(list_numbers) if n == 3] # List comprehension

[0, 3, 4, 8]

list_numbers = [3, 1, 2, 3, 3, 4, 5, 6, 3, 7, 8, 9, 10]
g = (i for i, n in enumerate(list_numbers) if n == 3) # Generator expression
print("Generators store values, the first value here is:", next(g), ". Then the next is:", next(g), "followed by ", next(g),"and finally ", next(g))

Generators store values, the first value here is: 0. Then the next is: 3 followed by  4 and finally  8

Generators are memory-efficient because they yield items one at a time instead of storing all results like list comprehensions.

Final Thoughts

Congrats, you have just learned about the index() function in Python! You have seen how it can help you work with lists. You have been introduced to some new concepts as well. For more on list comprehension, check out DataCamp's Python List Comprehension tutorial. DataCamp's Python Iterator Tutorial has more on generators and generator expressions.