> Antoon Pardon wrote: >> Ben Finney wrote: >> > Would it be better if every Enum instance had its own unique >> > subclass of EnumValue, that was used to instantiate values for >> > that enumeration? >> >> If you decide on keeping the current behaviour when comparing >> values of different enumerations, I would definitely answer >> yes. > > If values from different enums were to be compared for equality (and > not raise an exception), would it still be good to have those values > be of different types?
Yes, because people who would need a quick and dirty ordering could then use the type of the object as a first key. >> The only advise I have here is look at PEP 3000 that states: > >> Comparisons other than == and != between disparate types will raise >> an exception unless explicitly supported by the type. > >> Which seem to imply that == and != should not raise an exception but >> >, >=, <, <= should. > > This seems like a good guide for a policy on enumerations. > > How about this proposal: > > Enumerations (instances of Enum) contain a fixed sequence of unique, > arbitrary values. The values within an enumeration are instances of a > type (subclass of EnumValue), unique to that enumeration. > > Values from an enumeration can be compared for equality (__eq__, > __ne__) with any other value, and will only return True for the same > value from the same enumeration, otherwise False. Other comparison > operations can be performed only with values from the same > enumeration, otherwise an exception (EnumValueCompareError, subclass > of ValueError) is raised. Looks good to me. > This might result in the following simplistic implementation:: > > class EnumValue(object): > # ... > > def __eq__(self, other): > result = False > if self is other: > result = True > return result > > def __cmp__(self, other): > if isinstance(other, EnumValue): > if is_same_enumeration(self, other): > result = cmp(self.index, other.index) > else: > raise EnumValueCompareError(self, other) > return result > > This gives meaning to the "equal value" comparisons, but ensures that > other comparisons are errors. > > Comments so far? Isn't there an else missing? I have the impression that if you compare two enums from a different enumeration __cmp__ will raise UnboundLocalError. -- Antoon Pardon -- http://mail.python.org/mailman/listinfo/python-list
