How to Avoid Shared List References When Initializing Empty List Dictionaries in Python?

Initialization of Empty List Dictionary in Python

Attempting to create a dictionary of lists by using the fromkeys method may result in unexpected behavior where all dictionary keys are updated when appending to one key. This is because fromkeys creates a single list object and references it as the value for all keys.

To resolve this issue, use a dictionary comprehension:

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

This comprehension creates a new list object for each key, ensuring that each key has its own independent list.

Alternatively, in Python versions prior to 2.7, use a list comprehension passed to the dict constructor:

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

or, in Python 2.4-2.6, a generator expression can be passed to dict:

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

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