Why does hash(None) change across different platforms and in different calls?

Question

I'm facing a really weird behavior with the hash function on Python. When I run the following command on Mac OS (10.10) I get different values from different calls.

$ python -c "print hash(None)"
-9223372036579216774
$ python -c "print hash(None)"
-9223372036582852230

In the other hand when I run the same thing on Ubuntu 14.04 I get:

$ python -c "print hash(None)"
596615
$ python -c "print hash(None)"
596615

For me it looks like, in OS X, python is using the memory address somehow and Ubuntu is not. From that I can see that the hash function is probably implementation dependent. But shouldn't it be based on the "value" of None only? What do those numbers represent? Why does it behave differently even on the same python version but on different OS?

Answer 1

None.__hash__ correlates to the _Py_HashPointer hashing function. So basically the pointer of the object is used as hash. For None beeing a singleton this is safe to use, but not deterministic. For a pointer cast to an adequate integer type p the hash value is calculated like the following:

    (p >> 4) | (p << (8 * SIZEOF_VOID_P - 4))

Referring to this a comment in the source code states:

bottom 3 or 4 bits are likely to be 0; rotate y by 4 to avoid excessive hash collisions for dicts and sets