How to Avoid Shared List References When Creating a Dictionary of Empty Lists in Python?

Initializing a Dictionary of Empty Lists in Python

When attempting to create a dictionary of lists, you may encounter an unexpected issue where all keys in the dictionary share the same list reference. This occurs when using the fromkeys() method and passing an empty list as the second argument.

How to Use fromkeys() Properly

The dict.fromkeys() method takes two arguments: a collection of keys and a value. It creates a new dictionary with the provided keys, and each key is mapped to the specified value.

However, if an empty list [] is passed as the value, all keys in the resulting dictionary will be assigned the same list object. This means that any modifications to one key will also affect all other keys.

Alternative Solutions

To create a dictionary of distinct lists, consider using one of the following methods:

Dict Comprehension (Python 2.7 )

data = {k: [] for k in range(2)}

List Comprehension with dict() Constructor

data = dict([(k, []) for k in range(2)])

Generator Expression with dict() Constructor (Python 2.4-2.6)

data = dict((k, []) for k in range(2))

These methods ensure that each key in the dictionary is associated with a separate list object, allowing for individual key updates without affecting other keys.

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