In Python, no user object is modified by a simple assignment to a name. It just binds it.

It is not about mutable/immutable objects , it is about using a name for a single purpose within given scope.

    a = 1
    b = 2
    a = b

"a" name refers to the "2" object. "1" hasn't changed (ints are immutable in Python). If nothing else references it, it might as well disappear (or not).

Though both "single purpose" and immutability may be [distinct] good ideas.