How is hash(None) calculated?

Question

On my machine, hash(None) returns a value:

>>> hash(None)
-2138947203

Just out of curiosity, how is this hash value calculated? It doesn't seem as though this value is based on None's id as it is the same if I restart the Python interpreter.

Answer 1

It is based on None's id, but None is one of a few Python objects that are defined as C global variables, so its address (typically) doesn't change between Python runs. Other such objects are True and False (but these are hashed as ints), or built-in classes like object and tuple.

The address (and hash) is different between different CPython builds, however. On my system, hash(None) gives 539708.

Answer 2

It's based on the address of None in memory, as the type definition says.