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?
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
In other programming languages, what you are talking about are called "pointers."
They have this name because they "point" to things.
a points to b, b points to 2, and so on.
I have drawn some pictures to describe what you thought would happen, and what actually happened.
The variables in python are references. Few references are to immutable objects (such as strings, integers etc) whereas lists and sets are mutable.
Python 3.7.2 (tags/v3.7.2:9a3ffc0492, Dec 23 2018, 22:20:52) [MSC v.1916 32 bit (Intel)] on win32
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# Consider two variables x & y with integers 23 and 46. They have immutable references
# Meaning, if y=x, it doesn't mean y will change whenever x is updated.
>>> x=23
>>> print (x)
23
>>> y=46
>>> print (x)
23
>>> print (y)
46
>>> y=x
>>> print (x)
23
>>> print (y)
23
# Let's change the value of x after the assignment
>>> x=99
>>> print (x)
99
# Since it is immutable, the value wont change.
>>> print (y)
23
#Let's consider the mutable reference scenario. a & b are two lists which have mutable references
# Meaning, if b=a, it means b will change whenever a is changed
>>> a = list([11,22,33,87])
>>> print (a)
[11, 22, 33, 87]
>>> b = list([87,53,98,2])
>>> print (b)
[87, 53, 98, 2]
>>> a=b
>>> print (a)
[87, 53, 98, 2]
>>> print (b)
[87, 53, 98, 2]
# Popping the list would alter b
>>> b.pop()
2
>>> print (b)
[87, 53, 98]
# Notice the change in value of a
>>> print (a)
[87, 53, 98]
>>>
