Sets and Membership Tests

[Nick Vatamaniuc]

JK,
As a general rule, let Python call the "magic" __method__ methods
behind the scenes. So don't call obj.__hash()__ or obj.__len__ or
obj.__le__ just use hash(obj), len(obj) or <=. Of course there are
exceptions, for example when calling the __init__() method of a
supercalass inside the __init__ method of your class and perhaps a few
others...
Nick V.


JKPeck wrote:
> Thanks for the advice.  Once assured that __hash__ etc was the right
> route, I found that using hash() instead of object.__hash__() gave me
> stable hash valules.  (I am hashing strings that I know to be unique.)
>
> The "no luck" situation was that a set would accept the same object
> multiple times, not recognizing that it was truly the same object.
>
>
> Nick Vatamaniuc wrote:
> > JK,
> >
> > You are correct to implement __hash__ and __eq__. The problem is how
> > you implemented them. Usually your __eq__ method should compare the
> > necessary attributes of the objects for equality. The __hash__ should
> > return a 32-bit integer. Your best bet is probably to return a hash of
> > hashes of your attributes that are used in equality comparison. What
> > this means is that your attributes used to produce the __hash__ should
> > also be hashable. This is important yet not immediatly obvious. So you
> > could for example return hash( (attribute1, attribute2, attribute3) ),
> > where attribute1, attribute2, attribute3 are all hashable.
> >  Of course, you provided no code and no error messages (the
> > 'SoFarNoLuck' exception is not descriptive enough ; )
> >
> > Hope this helps
> >
> >
> >
> > JKPeck wrote:
> > > I would like to be able use sets where the set members are objects of a
> > > class I wrote.
> > > I want the members to be distinguished by some of the object content,
> > > but I have not figured out how a set determines whether two (potential)
> > > elements are identical.  I tried implementing __eq__ and __ne__ and
> > > __hash__ to make objects with identical content behave as identical for
> > > set membership, but so far no luck.
> > >
> > > I could subclass set if necessary, but I still don't know what I would
> > > need to override.
> > > 
> > > TIA for any advice you can offer.