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

Approaches to Initialize a Dictionary of Empty Lists in Python

Creating a dictionary of lists in Python can be achieved using various methods. However, if an attempt is made using the dict.fromkeys() method, one may encounter an issue where updating an individual key also modifies all other keys in the dictionary.

Understanding the dict.fromkeys() Method

The dict.fromkeys() method in Python initializes a new dictionary with the keys specified by the first argument. The value associated with each key is the second argument, which defaults to None. When an empty list [] is provided as the second argument, all values in the resulting dictionary refer to the same list object. This behavior leads to the unexpected result mentioned in the question.

Alternative Methods for Initialization

To avoid the aforementioned issue, alternative approaches can be employed:

1. Dict Comprehension (Python 2.7 and Higher)

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

This approach uses a dictionary comprehension to create a new dictionary where each key is mapped to an empty list.

2. List Comprehension (Python 2.4-2.6)

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

In this case, a list comprehension is used to generate a list of tuples, where each tuple consists of a key and an empty list. The list of tuples is then passed to the dict constructor to create the dictionary.

3. Generator Expression (Python 2.4-2.6)

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

Similar to the list comprehension approach, a generator expression can be used directly in the dict constructor, eliminating the need for surrounding parentheses.

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