I have just came up with an example of code:
ls = [2222, 1111]
a = ls[0]
print(a is ls[0])
a, ls[1] = ls[1], a
print(ls)
which prints:
True
[2222, 2222]
My question is, why isn't a the same object as ls[0] in the above case? The a is ls[0] check is True, therefore it must be the same as:
ls[0], ls[1] = ls[1], ls[0]
but it isn't. The latter one produces [1111, 2222]. What is this mYsTeRy?
Simple assignments have no affect on the previous value of the target.
After
ls = [2222, 1111]
a = ls[0]
both a and the first element of ls are references to the integer 2222.
The assignment
a, ls[1] = ls[1], a
is effectively the same as
t = ls[1], a # The tuple (1111, 2222)
a = t[0] # Set a to t[0] == 1111
ls[1] = t[1] # Set ls[1] to t[1] == 2222
At no point have you modified the first element of ls; you've only changed what a refers to and what the second element of ls refers to. You can see that a is now refers to 1111, since that's what the value of ls[1] was before ls[1] was modified.
>>> print(a)
1111