Python List Index Programiz

Kenji Sato

-Apr 7, 2026, 1:25 AM

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

How to use the index() method of a list - Implement a function like the find() method (returns-1 for non-existent values) - Get all indices of duplicate items - Specify the search range for the index() method - Use the index() method with tuples - Practical example: Find the index of the maximum/minimum value 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 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 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 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 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 To get all indices of a specific item, utilize the enumerate() function in conjunction with list comprehension.

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]) # [] 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] 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 i th to j th 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 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 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 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')) # [] 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]