How to Avoid Shared References When Creating Dictionaries of Empty Lists in Python?

Understanding the Challenges of Creating Dictionaries of Empty Lists

In Python, creating a dictionary of empty lists can be tricky due to the shared reference of the list object when using dict.fromkeys(). Attempting to append an element to one key, updates all keys with the same element due to the reference.

Fromkeys Method's Behavior

When the second argument to dict.fromkeys(), an empty list in this case, is passed, all values in the resulting dictionary share the same reference to the list object. Any modification made to one key affects all other keys.

Alternative Solutions

To avoid this issue, alternative solutions exist:

• Dict Comprehension (Python 2.7 and above):

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

This creates a dictionary where each key has its own unique, empty list.

• List Comprehension with Dict Constructor (Python versions before 2.7):

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

This achieves the same result as the dict comprehension by converting the list comprehension into a dictionary.

• Generator Expression with Dict Constructor (Python 2.4-2.6):

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

In this version, a generator expression is passed directly to the dict constructor, bypassing the list comprehension step. The parentheses surrounding the expression can be omitted.

By utilizing these alternative methods, dictionaries of empty lists can be created correctly, allowing manipulation of individual keys without affecting other keys.

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