Enumeration idioms: Values from different enumerations

[eswald at gmail.com]

Ben Finney wrote:
> Antoon Pardon wrote:
> > I just downloaded your enum module for python [from the Cheeseshop]
> > and played a bit with it. IMO some of the behaviour makes it less
> > usefull.
> [...]
> > I also think it would be more usefull if enums from different
> > enumerations just tested unequal.
> [...]
>
> However, I'm aware that this is not how other Python types behave:
>
>     >>> 23 == 23
>     True
>     >>> 42 == 23
>     False
>     >>> "spam" == 23
>     False
>
> Is there a semantic difference between these cases? Is that difference
> enough to want different behaviour?
>
> Is there some behaviour other than "evaluate to False" or "raise an
> exception", that could indicate "not comparable"?

Yes: return NotImplemented.  Note that the == operator automagically
returns False in this case.

    >>> "spam".__eq__("spam")
    True
    >>> "spam".__eq__("ham")
    False
    >>> "spam".__eq__(23)
    NotImplemented

This way, the user could explicitly call __eq__ and check for
NotImplemented if he desires the exceptional behavior.

- Eric