Hash functions

[Steven D'Aprano]

Do people often use hash() on built-in types? What do you find it useful
for?

How about on custom classes? Can anyone give me some good tips or hints
for writing and using hash functions in Python?

Thank you,

 

[Michael Hudson]

Do people often use hash() on built-in types?

Only implicitly.

What do you find it useful for?

Dictionaries

How about on custom classes?

Same here.

Can anyone give me some good tips or hints for writing and using
hash functions in Python?

Well, the usual tip for writing them is, don't, unless you need to.

If implement __eq__, then you need to, so it's fairly common to just
hash a tuple containing the things that are considered by the __eq__
method. Something like:

class C(object):
def __init__(self, a, b, c):
self.a = a
self.b = b
self.c = c
def __eq__(self, other):
return self.a == other.a and self.b == other.b
def __hash__(self):
return hash((self.a, self.b))

Cheers,
mwh