Comparison methods

[Rainer Deyke]

"Edw88253" <none at dev.null> wrote in message
news:tf46ngropc056b at news.supernews.com...
> I had two questions about Python's comparison methods (__cmp__, __eq__,
> etc.):
>
> ================================
> Question 1:
>
> When defining __eq__ for classes, is there any "standard" result when two
> objects are compared, and one's class is a superclass of the other?  I
ask,
> because initially I was defining __eq__ as:
>
> def __eq__(self, other):
>     if not isinstance(other, MyClass): return 0
>     return <appropriate expression>
>
> But, using that definition, a==b and b==a won't always return the same
> thing, which is presumably a Bad Thing.
>
> We could instead define __eq__ as:
>
> def __eq__(self, other):
>     if not isinstance(other, MyClass): return 0
>     if other.__class__ != MyClass: return 0
>     return <appropriate expression>

If you want different subclasses to compare as unequal, your code fails
where '__eq__' is inherited.  Try this instead:

  def __eq__(self, other):
    if type(other) != type(self) or other.__class__ != self.__class__:
      return 0
    return <appropriate expression>

There are cases where it makes more sense for instances of different
subclasses to sometimes compare as equal.  In these cases, your first
'__eq__' should work provided it is never overridden by a subclass.  I
cannot comment on whether your particular case is one of them.


--
Rainer Deyke (root at rainerdeyke.com)
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