On 2009-06-20 11:27, MRAB wrote:
Gustavo Narea wrote:
Hello again, everybody.
Thank you very much for your responses. You guessed right, I didn't
use the __hash__ method (and I forgot to mention that, sorry).
And unfortunately, I think I can't make them hashable, because the
objects are compared based on their attributes, which are in turn
other kind of objects compared based on other attributes. All these
class instances are compared with __eq__/__ne__ and they wrap
relatively complex data which would be hard to attempt to represent
them unambiguously using a 32-bit integer. That's why I'm afraid I
cannot use hashables.
I guess I'll have to use something like the function of my first
post. :(
A hash doesn't have to be unambiguous. It's just a way to reduce the
number of equality checks that have to be made.
Could you create a hash from a tuple of attributes?
A typical way to implement this is to make a method that returns a tuple of
attributes to use in both the __eq__ and __hash__ methods.
class Foo(object):
def key(self):
return (self.x, self.y, self.bar)
def __eq__(self, other):
return self.key() == other.key()
def __ne__(self, other):
return not (self == other)
def __hash__(self):
return hash(self.key())
As long as those attributes are also hashable, this usually works well.
--
Robert Kern
"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco
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