When creating lists, it seems that forms like [[False] * 3] * 3 only copies the references?

Question

I create a list using the below codes.

a = [[False] * 3] * 3

which creates a 3x3 matrix with all elements of False value.

And when I changed a[0][0] to be True using a[0][0] = True, the result is:

[[True, False, False], [True, False, False], [True, False, False]]

where the entire first column changes into True.

However, if I create a using the for loop, like:

a = [[False] * 3 for i in range(3)]

or

a = [[False for i in range(3)] for i in range(3)]

And after executing the same a[0][0] = True, I get my desired result in both ways as:

[[True, False, False], [False, False, False], [False, False, False]]

where just one element is changed into True.

Why there is such difference in creating the list in Python? Thank you for answering!

kwkt · Accepted Answer · 2021-01-15T01:44:08.027

1

[False] * 3 gives us a list of 3 references of the same False, but since boolean is immutable, each one of those is independent.
[[False] * 3] * 3 gives us a list of 3 references of the same [False] * 3, and since list is mutable, those 3 lists are actually the same.
a = [[False] * 3 for i in range(3)] is equivalent to:

a = []
for i in range(3):
    a.append([False] * 3)

That should tells you why the lists are different.

For [[False for i in range(3)] for i in range(3)], the only difference from above is that the Falses are created independently, but since boolean is immutable anyway, the result is identical to the above.

edited Jan 15 '21 at 01:44

answered Jan 15 '21 at 01:35

kwkt

"3 copies of" is inaccurate. It's the same object 3 times - or in other words, 3 references to the same object. This is true regardless of whether the object is mutable. – user2357112 Jan 15 '21 at 01:42
@user2357112supportsMonica That's true, let me edit my answer. – kwkt Jan 15 '21 at 01:43

1 Answers1