TL;DR: do not use list multiplication to initialize an empty list of list, or you will end up wasting hours debugging your program.

I wrote some Python code for my project and found that the result isn’t correct. So I spent quite some time debugging the whole working process of this program, and found sadly that the culprit is a list of empty list, which is wrongly initialized.

What is the resulting list, if we do the following in Python?

x = [[]] * 3
x[0].append(1.0)

I expect x now becomes [[1.0], [], []]. Instead, it becomes [[1.0], [1.0], [1.0]]. This is because when we use multiplication to create list x, we actually created 3 references to an empty list. List is a mutable object in Python. When we append values to a list, we haven’t changed its identity. As a result, changing either one of the 3 sub-list will change the others, since they all refer to the same list.

Note that things are different when we create a list of same immutable objects using multiplication. For example, if we create a list of same int, and then change one of the them, the other elements will not change, because we can not change the value of immutable types, assigning a new int to a list element will make it point to another address.

In [18]: a = [1] * 2

In [19]: a
Out[19]: [1, 1]

In [20]: print(id(a[0]), id(a[1]))
4438635584 4438635584

In [21]: a[0] = 2

In [22]: print(id(a[0]), id(a[1]))
4438635616 4438635584

To create a list 3 empty lists, we may use list comprehension instead:

x = [[] for _ in range(3)]

In each iteration, a new empty list is created. So the 3 sub-list are independent of each other. Changing one won’t affect the others.

References#

• List of lists changes reflected across sublists unexpectedly
• https://bugs.python.org/issue27135
• https://docs.python.org/3/faq/programming.html#how-do-i-create-a-multidimensional-list