Why Does This Code for Initializing a List of Lists Apparently Link the Lists?

When attempting to initialize a list of lists with a specific length, using the syntax x = [[]] * n may result in unexpected behavior where the lists appear to be linked together.

This issue arises because the star (*) operator used in [[]] * n creates a list of references to the same underlying list object. When you modify one of the lists in the sequence, the changes are reflected in all of the other lists because they all refer to the same object.

Solution

To create a list of distinct lists, it's important to initialize each list separately. This can be achieved using the following syntax:

x = [[] for i in range(n)]

In this case, the list comprehension creates a new list for each iteration of the loop, resulting in a list of independent list objects.

Comparison

To illustrate the difference, consider the following code snippet:

x = [[]] * 3
x[1].append(0)
print(x)  # Output: [[0], [0], [0]]

In this example, the list x contains three references to the same list object, so any modifications made to one list affect the others.

On the other hand, the following code snippet creates a list of three distinct lists:

x = [[] for i in range(3)]
x[1].append(0)
print(x)  # Output: [[], [0], []]

Here, each list in x is a separate object, so changes made to one list do not affect the others.