Why are these tuples returned from a function identical?

Question

Let's say I have a function:

def get_tuple():
    return (1,)

In IDLE if I call:

get_tuple() is get_tuple()

It will print True

If I call:

(1,) is (1,)

It will print False

Do you know why? I cannot find an explanation in the Python documentation.

Answer 1

The CPython implementation of Python stores constant values (such as numbers, strings, tuples or frozensets) used in a function as an optimization. You can observe them via func.__code__.co_consts:

>>> def get_tuple():
...     return (1,)
...
>>> get_tuple.__code__.co_consts
(None, 1, (1,))

As you can see, for this function the constants None (which is the default return value), 1 and (1,) get stored. Since the function returns one of them, you get the exact same object with every call.

Because this is a CPython implementation detail, it's not guaranteed by the Python language. That's probably why you couldn't find it in the documentation :)