How to Properly Initialize a Dictionary with Empty Lists in Python?

Dictionary Initialization with Empty Lists in Python

In Python, creating a dictionary of empty lists can be achieved through various approaches. However, the fromkeys method may not produce the desired outcome as it creates a single list object referenced by all dictionary keys.

Consider the following example:

data = {}
data = data.fromkeys(range(2), [])
data[1].append('hello')
print(data)

Expected Result: {0: [], 1: ['hello']}
Actual Result: {0: ['hello'], 1: ['hello']}

Cause:

When using fromkeys, the second argument ([]) is treated as a single list object, which is referenced by all keys in the resulting dictionary. Any modifications to any key will propagate to all others.

Solution:

To create a dictionary of independent empty lists, use a dict comprehension:

In Python 2.7 or above:

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

In Python versions prior to 2.7:

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

Alternatively, in Python 2.4 - 2.6, use a generator expression:

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

These approaches ensure that each key in the dictionary references a distinct empty list object, allowing for independent modifications and addressing of dictionary values.

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