How is the return value of __hash__ used?

Question

Suppose I write a class, but don\'t define a __hash__ for it. Then __hash__(self) defaults to id(self) (self\'s memory addres

Answer 1

Since Python's hash tables have a size that is a power-of-two, the lower bits of the hash value determine the location in the hash table (or at least the location of the initial probe).

The sequence of probes into a table size of n is given by:

def gen_probes(hashvalue, n):
    'Same sequence of probes used in the current dictionary design'
    mask = n - 1
    PERTURB_SHIFT = 5
    if hashvalue < 0:
        hashvalue = -hashvalue
    i = hashvalue & mask
    yield i
    perturb = hashvalue
    while True:
        i = (5 * i + perturb + 1) & 0xFFFFFFFFFFFFFFFF
        yield i & mask
        perturb >>= PERTURB_SHIFT

For example, the dictionary:

d = {'timmy': 'red', 'barry': 'green', 'guido': 'blue'}

is stored as an array of size 8 with each entry in the form (hash, key, value):

entries = [['--', '--', '--'],
           [-8522787127447073495, 'barry', 'green'],
           ['--', '--', '--'],
           ['--', '--', '--'],
           ['--', '--', '--'],
           [-9092791511155847987, 'timmy', 'red'],
           ['--', '--', '--'],
           [-6480567542315338377, 'guido', 'blue']]

The C source code for key insertion in Python's dictionaries can be found here: http://hg.python.org/cpython/file/cd87afe18ff8/Objects/dictobject.c#l550