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Find the Index of an Item in a List in Python

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In Python, the index() method allows you to find the index of an item in a list.

Contents

How to use the index() method of a list

To find the index of an item in a list, specify the desired item as an argument to the index() method. This method returns the zero-based index of the item.

l = [30, 50, 10, 40, 20]

print(l.index(30))
# 0

print(l.index(20))
# 4
source: list_index.py

If the specified item is not present in the list, the index() method will throw a ValueError.

# print(l.index(100))
# ValueError: 100 is not in list
source: list_index.py

Implement a function like the find() method (returns -1 for non-existent values)

String objects (str) have a find() method that returns -1 if a substring is not present. However, no such method exists for lists (as of Python 3.11).

To create a function that emulates the behavior of the find() method for lists, use the in operator to check if an item is in the list.

def my_find(l, x):
    if x in l:
        return l.index(x)
    else:
        return -1

l = [30, 50, 10, 40, 20]

print(my_find(l, 30))
# 0

print(my_find(l, 100))
# -1
source: list_index.py

You can write it more simply using a ternary operator.

def my_find2(l, x):
    return l.index(x) if x in l else -1

print(my_find2(l, 30))
# 0

print(my_find2(l, 100))
# -1
source: list_index.py

Get all indices of duplicate items

The index() method will only return the index of the first instance of an item if it appears multiple times in a list.

l = [10, 30, 10, 10, 20, 20]

print(l.index(10))
# 0

print(l.index(20))
# 4
source: list_index.py

To get all indices of a specific item, utilize the enumerate() function in conjunction with list comprehension.

This operation returns an empty list if the item does not exist in the list.

print([i for i, x in enumerate(l) if x == 10])
# [0, 2, 3]

print([i for i, x in enumerate(l) if x == 20])
# [4, 5]

print([i for i, x in enumerate(l) if x == 30])
# [1]

print([i for i, x in enumerate(l) if x == 100])
# []
source: list_index.py

The process can be defined as a function as follows:

def my_index_multi(l, x):
    return [i for i, _x in enumerate(l) if _x == x]

print(my_index_multi(l, 10))
# [0, 2, 3]
source: list_index.py

Refer to the following articles if you want to remove or extract duplicate elements from a list.

Specify the search range for the index() method

The index() method supports optional second and third arguments i and j, allowing you to specify a search range from the ith to jth elements (with j exclusive). If j is not provided, the search continues to the end of the list.

Regardless of the range specified, the returned index is relative to the start of the full list.

l = [10, 30, 10, 10, 20, 20]

print(l.index(10))
# 0

print(l.index(10, 2))
# 2

# print(l.index(20, 2, 4))
# ValueError: 20 is not in list

print(l.index(20, 2, 5))
# 4
source: list_index.py

While l.index(x, i, j) and l[i:j].index(x) target the same range, the index returned when using slices is relative to the start of the slice.

print(l[2:])
# [10, 10, 20, 20]

print(l[2:].index(10))
# 0

print(l[2:5])
# [10, 10, 20]

print(l[2:5].index(20))
# 2
source: list_index.py

Use the index() method with tuples

Like lists, tuples also have the index() method.

t = ('c', 'a', 'a', 'b', 'c')

print(t.index('a'))
# 1

# print(t.index('x'))
# ValueError: tuple.index(x): x not in tuple
source: list_index.py

The function defined above can be directly applied to tuples as well.

print(my_find(t, 'a'))
# 1

print(my_find(t, 'x'))
# -1

print(my_index_multi(t, 'a'))
# [1, 2]

print(my_index_multi(t, 'x'))
# []
source: list_index.py

Practical example: Find the index of the maximum/minimum value

As a practical example, find the index of the maximum or minimum value in a list.

You can find the index of the maximum or minimum value in a list by passing the result of max() or min() to the index() method.

You can also use the function defined above.

l = [10, 30, 10, 10, 20, 20]

print(l.index(max(l)))
# 1

print(l.index(min(l)))
# 0

print(my_index_multi(l, max(l)))
# [1]

print(my_index_multi(l, min(l)))
# [0, 2, 3]
source: list_index.py

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