What happens when you assign the value of one variable to another variable in Python?

Question

This is my second day of learning python (I know the basics of C++ and some OOP.), and I have some slight confusion regarding variables in python.

Here is how I understand them currently:

Python variables are references (or pointers?) to objects (which are either mutable or immutable). When we have something like num = 5, the immutable object 5 is created somewhere in memory, and the name-object reference pair num is created in a certain namespace. When we have a = num, nothing is being copied, but now both variables refer to the same object and a is added to the same namespace.

This is where my book, Automate the boring stuff with Python, confuses me. As it's a newbie book, it doesn't mention objects, namespaces, etc., and it attempts to explain the following code:

>>> spam = 42 >>> cheese = spam >>> spam = 100 >>> spam 100 >>> cheese 42 

The explanation it offers is exactly the same as that of a C++ book, which I am not happy about as we are dealing with references/pointers to objects. So in this case, I guess that in the 3rd line, as integers are immutable, spam is being assigned an entirely new pointer/reference to a different location in memory, i.e. the memory that it was initially pointing to wasn't modified. Hence we have cheese referring to the initial object referred to by spam. Is this the correct explanation?

Answer 1

As a C++ developer you can think of Python variables as pointers.

Thus when you write spam = 100, this means that you "assign the pointer", which was previously pointing to the object 42, to point to the object 100.

Earlier on, cheese was assigned to point to the same object as spam pointed to, which happened to be 42 at that time. Since you have not modified cheese, it still points to 42.

Immutability has nothing to do with it in this case, since pointer assignment does not change anything about the object being pointed to.

Answer 2

It is correct you can more or less thing of variables as pointers. However example code would help greatly with explaining how this actually is working.

First, we will heavily utilize the id function:

Return the “identity” of an object. This is an integer which is guaranteed to be unique and constant for this object during its lifetime. Two objects with non-overlapping lifetimes may have the same id() value.

It's likely this will return different absolute values on your machine.

Consider this example:

>>> foo = 'a string' >>> id(foo)  4565302640 >>> bar = 'a different string' >>> id(bar) 4565321816 >>> bar = foo >>> id(bar) == id(foo) True >>> id(bar) 4565302640 

You can see that:

• The original foo/bar have different ids, because they point to different objects
• When bar is assigned to foo, their ids are now the same. This is similar to them both pointing to the same location in memory that you see in making a C++ pointer

when we change the value of foo, it is assigned to a different id:

>>> foo = 42 >>> id(foo) 4561661488 >>> foo = 'oh no' >>> id(foo) 4565257832 

An interesting observation too is that integers implicitly have this functionality up to 256:

>>> a = 100 >>> b = 100 >>> c = 100 >>> id(a) == id(b) == id(c) True 

However beyond 256 this is no longer true:

>>> a = 256 >>> b = 256 >>> id(a) == id(b) True >>> a = 257 >>> b = 257 >>> id(a) == id(b) False 

however assigning a to b will indeed keep the id the same as shown before:

>>> a = b >>> id(a) == id(b) True