Different behaviour for list.__iadd__ and list.__add__

consider the following code:

>>> x = y = [1, 2, 3, 4]
>>> x += [4]
>>> x
[1, 2, 3, 4, 4]
>>> y
[1, 2, 3, 4, 4]

and then consider this:

>>> x = y = [1, 2, 3, 4]
>>> x = x + [4]
>>> x
[1, 2, 3, 4, 4]
>>> y
[1, 2, 3, 4]

Why is there a difference these two?

(And yes, I tried searching for this).

python list

Edit Report

Different behaviour for list.iadd and list.add