On Mon, Oct 12, 2009 at 3:12 PM, Peng Yu <pengyu...@gmail.com> wrote: > On Mon, Oct 12, 2009 at 4:03 PM, Robert Kern <robert.k...@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__?
No; dictionaries are unordered and thus don't utilize the non-equality comparison operators. Cheers, Chris -- http://blog.rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
