How Can I Handle Duplicate Keys When Creating a Python Dictionary?

Duplicate Keys in Python Dictionaries

In the programming realm, dictionaries prove invaluable for organizing data by mapping unique keys to corresponding values. However, the standard Python dictionary implementation adheres to a strict principle: each key must remain unique within the dictionary. This constraint poses a challenge when dealing with scenarios involving duplicate keys in a dataset.

Creating a Dictionary with Duplicate Keys

Suppose we encounter a text file containing duplicate car registration numbers, each associated with different information such as driver names, phone extensions, and parking locations. The task is to construct a dictionary with registration numbers as keys and the corresponding data as values. The following snippet illustrates a common approach:

However, this code faces a limitation due to Python's default dictionary behavior: duplicate keys result in overwriting previous values. To circumvent this, a solution is to store lists or sets within the dictionary.

Using Collections.defaultdict

The Python standard library provides a useful tool for this purpose: collections.defaultdict. This class initializes a dictionary with a default factory, ensuring that a non-existent key will create a new list or set (depending on the factory argument).

To populate the dictionary with duplicate keys, simply replace the original assignment statement:

with:

This strategy effectively creates a dictionary of lists, where each key corresponds to a list of associated values. By leveraging collections.defaultdict, you can successfully handle duplicate keys in your Python dictionaries.

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