On Mon, Oct 12, 2009 at 4:03 PM, Robert Kern <robert.kern at gmail.com>
wrote:> On 2009-10-12 15:45 PM, Peng Yu wrote:
>>
>> Hi,
>>
>> I'm wondering what is the general way to define __hash__. I could
add
>> up all the members. But I am wondering if this would cause a
>> performance issue for certain classes.
>
> Unless if you are very familiar with the math of hash functions, I
don't
> recommend that you try to implement one directly. Instead, make a tuple of
> the hashable content of your class and return the result of calling hash()
> on that tuple. Be sure to make your equality comparison do the right thing.
>
> class A(object):
> ?def __init__(self, a, b):
> ? ?self.a = a
> ? ?self.b = b
>
> ?def _key(self):
> ? ?# I include the name of the class so as to differentiate between other
> ? ?# classes that might also have a _key() method. If you have several
> classes
> ? ?# or subclasses that are allowed to compare equal to each other, use
some
> ? ?# other common string here.
> ? ?return (type(self).__name__, a, b)
>
> ?def __hash__(self):
> ? ?return hash(self._key())
>
> ?# Coincidentally, the _key() method can usually be reused for comparisons.
> ?# I recommend doing this for the equality comparisons, at least, when you
> can
> ?# because of the requirement that two items that compare equal must have
> the
> ?# same hash value.
> ?def __eq__(self, other):
> ? ?return self._key() == other._key()
>
> ?def __ne__(self, other):
> ? ?return not (self == other)
>
> ?...
Do I need to define other 4 comparison operators besides __eq__ and __ne__?
