How to Find Indices of Duplicate Items in a Python List

This guide explains how to find the indices of duplicate items within a Python list. We'll cover two main scenarios: finding all indices of a given value, and finding indices starting from a specific position within the list. We'll use enumerate() and list comprehensions for concise and efficient solutions.

Finding All Indices of a Value

The most efficient and Pythonic way to find all indices of a specific value in a list is to use a list comprehension with enumerate():

def find_indices(l, value):
    return [index for index, item in enumerate(l) if item == value]

# Example Usage:
my_list = ['one', 'two', 'one', 'one']
print(find_indices(my_list, 'one'))  # Output: [0, 2, 3]
print(find_indices(my_list, 'abc'))  # Output: [] (empty list if not found)

• enumerate(l): This function adds a counter to an iterable and returns it (as an enumerate object). It produces pairs of (index, item) for each item in the list l.
• for index, item in enumerate(l): This loop unpacks each pair from enumerate() into the index and item variables.
• if item == value: This is the filtering condition. We only keep the index if the item matches the value we're looking for.
• [index ... ]: This is the list comprehension. It builds a new list containing only the index values where the condition is true.
• Empty List for No Matches: If the value is not found in the list, the list comprehension produces an empty list ([]). This is a good, clean way to indicate no matches, and it avoids raising an exception.

Finding Indices Starting from a Specific Position

To find indices of a value, but starting the search from a particular position in the list, you can use a while loop and list.index(), but a generator function is often cleaner and more flexible:

def find_indices(l, value, start=0):
    indices = []
    while True:
        try:
            index = l.index(value, start) # Find the index
            start = index + 1 # Increment start index
            indices.append(index) # Append the index
        except ValueError:
            break # Stop if item not in the list
    return indices

# Example Usage:
my_list = ['a', 'b', 'a', 'a', 'b', 'a']

print(find_indices(my_list, 'a', 1))    # Output: [2, 3, 5] (start from index 1)
print(find_indices(my_list, 'a', 3))    # Output: [3, 5]    (start from index 3)
print(find_indices(my_list, 'a', 4))    # Output: [5]       (start from index 4)
print(find_indices(my_list, 'a', 6))    # Output: []        (start from index 6, no more 'a's)
print(find_indices(my_list, 'x'))       # Output: []

• l.index(value, start): The key here is the start parameter of the index() method. It tells index() to begin searching from that position, not from the beginning of the list. If the value isn't found after the start index, index() raises a ValueError.
• while True and try/except ValueError: This structure handles the case where the value is not found (or no longer found after the start index). The ValueError breaks the loop. This is a standard Python idiom for looping until a condition is met.
• start = index + 1: This is crucial. We update start to be after the index we just found, so the next call to index() doesn't find the same item again.
• The function will always return a list of indexes.