Understanding Python's “is” operator

Question

The is operator does not match the values of the variables, but the instances themselves.

What does it really mean?

Answer 1

is and is not are the two identity operators in Python. is operator does not compare the values of the variables, but compares the identities of the variables. Consider this:

>>> a = [1,2,3]
>>> b = [1,2,3]
>>> hex(id(a))
'0x1079b1440'
>>> hex(id(b))
'0x107960878'
>>> a is b
False
>>> a == b
True
>>>

The above example shows you that the identity (can also be the memory address in Cpython) is different for both a and b (even though their values are the same). That is why when you say a is b it returns false due to the mismatch in the identities of both the operands. However when you say a == b, it returns true because the == operation only verifies if both the operands have the same value assigned to them.

Interesting example (for the extra grade):

>>> del a
>>> del b
>>> a = 132
>>> b = 132
>>> hex(id(a))
'0x7faa2b609738'
>>> hex(id(b))
'0x7faa2b609738'
>>> a is b
True
>>> a == b
True
>>>

In the above example, even though a and b are two different variables, a is b returned True. This is because the type of a is int which is an immutable object. So python (I guess to save memory) allocated the same object to b when it was created with the same value. So in this case, the identities of the variables matched and a is b turned out to be True.

This will apply for all immutable objects:

>>> del a
>>> del b
>>> a = "asd"
>>> b = "asd"
>>> hex(id(a))
'0x1079b05a8'
>>> hex(id(b))
'0x1079b05a8'
>>> a is b
True
>>> a == b
True
>>>

Hope that helps.