On 2009-10-12 17:12 PM, Peng Yu 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__?
Yes. There are recipes to fill in the rest if you just provide __eq__ and
__lt__:
http://code.activestate.com/recipes/576685/
Or you could continue to use the __cmp__ method, but it will be going away in
Python 3.
--
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
--
http://mail.python.org/mailman/listinfo/python-list
