I cannot understand aliases, apparently

[jefsummers]

You have reassigned what a points to, so a no longer is b. Here's a longer interchange that might make it more clear.

>>> a=3
>>> b=a
>>> a is b
True
>>> c = 4
>>> a = c
>>> c is a
True
>>> a is b
False
>>> 

When a is reassigned to c it is no longer linked to b.
Reminds me of quantum entanglement ;^).