Variable assignment and modification with regards to memory addresses

Question

Suppose I assign two variables to integers:

a = 1
b = 2

Now, I'll assign a to b:

a = b

As expected, a == 2, because a has been set to the memory address of b.

But actually, it hasn't. If I do

b += 1

a still equals 2. Why does a not point to b?

Answer 1

The behavior in the example is as follows

In [1]: a = 1                                                                                                                                                                     

In [2]: b = 2                                                                                                                                                                     

In [3]: a = b                                                                                                                                                                     

In [4]: b+=1                                                                                                                                                                      

In [5]: b                                                                                                                                                                         
Out[5]: 3

In [6]: a                                                                                                                                                                         
Out[6]: 2

In the example, when you do a=b, both a and b are pointing to the same reference, but when you b += 1, the operation of adding 1 to b, creates a new integer value 3 for b and b points to that value, but a is still pointing to the old value 2

Note that trying to do it with mutable types like a list works as what you were excepting would happen to an integer

In [1]: a = [1]                                                                                                                                                                   

In [2]: b = [2]                                                                                                                                                                   

In [3]: a = b                                                                                                                                                                     

In [4]: b.append(2)                                                                                                                                                               

In [5]: a                                                                                                                                                                         
Out[5]: [2, 2]

In [6]: b                                                                                                                                                                         
Out[6]: [2, 2]

In [7]: b += [3, 4];                                                                                                                                                                        

In [8]: b
Out[8]: [2, 2, 3, 4]

In [9]: a
Out[9]: [2, 2, 3, 4]

Now what happened here? We changed b but a changed as well, this is because the append or update of the list happens in-place and since a was pointing to b both ends up getting updated!

What happens for += operator is defined by __iadd__ method of a class. For int-s all the __iXXX__ methods return a new instance of int. For list-s __iadd__(self, other) does self.extend(other); return self, so variable keeps pointing to same object.

As a result, even integers can be made to behave as a list, an example is here courtesy @imposeren