How to Find the Most Common Element in a Python List, Even If They're Not Hashable?

Finding the Most Common Element in a Python List

In Python, identifying the most commonly occurring element in a list is a common programming task. However, it becomes a challenge when the elements are not hashable, preventing the use of dictionaries.

Solution

One efficient solution involves utilizing the max function with a custom key. The max function typically compares elements based on their intrinsic properties, but we can redefine this behavior by providing a custom key to facilitate comparison based on the frequency of the elements.

Here's a concise implementation in Python:

def most_common(lst):
    return max(set(lst), key=lst.count)

This solution first creates a set from the input list, which automatically eliminates duplicates while preserving the order of the elements. Then, the max function is applied to this set, using the lst.count method as the key. The lst.count method returns the number of occurrences of each element in the original list. By using this method as the key, the max function effectively ranks the elements based on their frequencies.

In case of ties, the max function will select the element with the lowest index in the original list. This behavior ensures that, in the given example, most_common(['duck', 'duck', 'goose']) will correctly return 'duck'.

This solution offers a simple and efficient way to find the most common element in a Python list, even if the elements are not hashable.

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