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

It compares object identity, that is, whether the variables refer to the same object in memory. It's like the == in Java or C (when comparing pointers).

Answer 2

X points to an array, Y points to a different array. Those arrays are identical, but the is operator will look at those pointers, which are not identical.

Answer 3

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]

Answer 4

x is y is same as id(x) == id(y), comparing identity of objects.

As @tomasz-kurgan pointed out in the comment below is operator behaves unusually with certain objects.

E.g.

>>> class A(object):
...   def foo(self):
...     pass
... 
>>> a = A()
>>> a.foo is a.foo
False
>>> id(a.foo) == id(a.foo)
True

Ref;
https://docs.python.org/2/reference/expressions.html#is-not
https://docs.python.org/2/reference/expressions.html#id24

Answer 5

A simple example with fruits

fruitlist = [" apple ", " banana ", " cherry ", " durian "]
newfruitlist = fruitlist
verynewfruitlist = fruitlist [:]
print ( fruitlist is newfruitlist )
print ( fruitlist is verynewfruitlist )
print ( newfruitlist is verynewfruitlist )

Output:

True
False
False

If you try

fruitlist = [" apple ", " banana ", " cherry ", " durian "]
newfruitlist = fruitlist
verynewfruitlist = fruitlist [:]
print ( fruitlist == newfruitlist )
print ( fruitlist == verynewfruitlist )
print ( newfruitlist == verynewfruitlist )

The output is different:

True
True
True

That's because the == operator compares just the content of the variable. To compare the identities of 2 variable use the is operator

To print the identification number:

print ( id( variable ) )

Answer 6

Prompted by a duplicate question, this analogy might work:

# - Darling, I want some pudding!
# - There is some in the fridge.

pudding_to_eat = fridge_pudding
pudding_to_eat is fridge_pudding
# => True

# - Honey, what's with all the dirty dishes?
# - I wanted to eat pudding so I made some. Sorry about the mess, Darling.
# - But there was already some in the fridge.

pudding_to_eat = make_pudding(ingredients)
pudding_to_eat is fridge_pudding
# => False