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 only returns true if they're actually the same object. If they were the same, a change to one would also show up in the other. Here's an example of the difference.

>>> x = [1, 2, 3]
>>> y = [1, 2, 3]
>>> print x is y
False
>>> z = y
>>> print y is z
True
>>> print x is z
False
>>> y[0] = 5
>>> print z
[5, 2, 3]