Difference between list defined with multiplication and without

Question

Is there an explainer on the difference between?

aL, bL = [[], []]
xL, yL = [[]] * 2

Superficially, they should be the same.

[[], []] == [[]] * 2
True

But they behave differently.

aL.append("a")
xL.append("x")
bL
[]
yL
['x']

Answer 1

>>> [[]] * 2 == [[], []] # True

>>> a1, b1 = [[], []]
>>> id(a1) # 4425627680
>>> id(b1) # 4425627760

>>> a2, b2 = [[]] * 2
>>> id(a2) # 4424875616
>>> id(b2) # 4424875616

When you utilize the * expression to generate a list of lists, Python is copying the inner object of [[]], and copying its reference over. Therefore, when doing [[]] * 2, you end up with a list of lists in which all of the inner lists are the same object. If you are still confused, I recommend checking out this post.

Now why [[]] * 2 == [[], []] is True, this is easy to observe:

>>> bool([[], []]) # True
>>> bool([[]] * 2) # True
>>> bool([]) # False

A list with anything inside always evaluates to True, therefore you are doing True == True which is of course True.