How to Count Duplicates in a Python List

Here are 2 ways to count duplicates in a Python List:

1. Using the “count” method:

my_list = ["a", "a", "z", "z", "s", "a", "a", "b", "b", "k"]

my_dict = {i: my_list.count(i) for i in my_list}

print(my_dict)

The result:

{'a': 4, 'z': 2, 's': 1, 'b': 2, 'k': 1}

2. Using collections:

import collections

my_list = ["a", "a", "z", "z", "s", "a", "a", "b", "b", "k"]

my_dict = dict(collections.Counter(my_list))

print(my_dict)

The result:

{'a': 4, 'z': 2, 's': 1, 'b': 2, 'k': 1}

Keeping only the duplicates

To keep only the duplicates (while filtering out the non-duplicate values):

import collections

my_list = ["a", "a", "z", "z", "s", "a", "a", "b", "b", "k"]

counter = collections.Counter(my_list)

my_dict = {k: v for k, v in counter.items() if v > 1}

print(my_dict)

The result:

{'a': 4, 'z': 2, 'b': 2}

Create a function

Here is a function to count duplicates in a list using collections:

import collections


def count_duplicates(my_list: list) -> dict:
    """
    Function to count duplicates values in a list

    :param my_list: The input list to count the duplicates
    :return: The dictionary that contains the duplicates
    """
    counter = collections.Counter(my_list)

    return {k: v for k, v in counter.items() if v > 1}


example_list = ["a", "a", "z", "z", "s", "a", "a", "b", "b", "k"]

print(count_duplicates(example_list))

The result:

{'a': 4, 'z': 2, 'b': 2}

Test the function

To test the “count_duplicates” function using Pytest:

import pytest

from app.list_operations import count_duplicates


@pytest.mark.parametrize(
    "input_list, expected_output",
    [
        (["a", "a", "z", "z", "s", "a", "a", "b", "b", "k"], {"a": 4, "z": 2, "b": 2}),
        (["a", "a"], {"a": 2}),
        (["a"], {}),
        (["aa", "aa", "bbb", "bb", "cc", "cc"], {"aa": 2, "cc": 2}),
        ([1, 1, 3, 1, 5, 5, 9], {1: 3, 5: 2}),
    ],
)
def test_count_duplicates(input_list, expected_output):
    assert count_duplicates(input_list) == expected_output

The result:

============================= test session starts =============================
collecting ... collected 5 items

test_list_operations.py::test_count_duplicates[input_list0-expected_output0] PASSED [ '20%']
test_list_operations.py::test_count_duplicates[input_list1-expected_output1] PASSED [ '40%']
test_list_operations.py::test_count_duplicates[input_list2-expected_output2] PASSED [ '60%']
test_list_operations.py::test_count_duplicates[input_list3-expected_output3] PASSED [ '80%']
test_list_operations.py::test_count_duplicates[input_list4-expected_output4] PASSED ['100%']

============================== 5 passed in '0.01s' ==============================
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