On 29/04/2013 19:24, Steven D'Aprano wrote: > On 30/04/13 03:01, Guido van Rossum wrote: > >> Oh dear, this is actually a mess. I don't want MoreColor.red and >> Color.red to be distinct objects, but then the isinstance() checks >> will become confusing. If we don't override isinstance(), we'll >> get >> >> not isinstance(Color.red, MoreColor) isinstance(MoreColor.yellow, >> Color) >> >> This would be pretty backwards. > > Why is that backwards? MoreColor is a subclass of Color, so instances > of MoreColor are instances of Color, but instances of Color are not > instances of MoreColor. That's normal behaviour for subclasses. (All > cats are mammals, but not all mammals are cats.) > Let's say that Color is red, green, or blue. Let's also say that MoreColor is superset of Color, in other words, any of Color, plus cyan, magenta, or yellow. Red is a Color and a MoreColor (member of Color and MoreColor). Yellow is a MoreColor, but not a Color (member of MoreColor but not Color). That's the opposite of the rules of inheritance. > The only "funny" thing about this is that enumeration values of a > Enum are only instances of that Enum if they are declared inside that > Enum, and as far as I'm concerned that's not especially funny at all. > So: > > Color declares red, so Color.red is a Color. > > MoreColor does not declare red, it merely inherits it from Color, so > MoreColor.red is Color.red which is not a MoreColor. > > > This leads to a simple rule: > > Enum values are either instances of the enum they are defined in, or > instances of the parent class they are inherited from. > > If the user doesn't like that, they're free to not subclass Enums > :-) > > > >> I Ggoogled "enum subclassing" and found this StackOverflow article >> explaining why you can't subclass enums in Java: >> http://stackoverflow.com/questions/4604978/subclassing-an-enum >> which refers to this more elaborate answer: >> http://stackoverflow.com/questions/3427947/enumeration-inheritence-in-java/3428050#3428050 > >> > The first link says: > > "And generally, a proper subclass is a specialization of its > superclass." > > and gives the Liskov Substitution Principle as the reason for > prohibiting subclassing. LSP is a very useful principle, but it is > not the only model for subclassing. That's why it's called a > *principle* and not the Liskov Substitution *Law*. > > (Yes, I stole that from Raymond Hettinger's talk on subclassing at > PyCon.) > > I think that the ability to extend enums with new ones, without > duplicating code, is more important for enums than satisfying LSP. > The common use-cases for enums do not lend itself to subtyping in the > Liskov sense. > > E.g. if I have a function that expects a Color red/blue/green enum, > and I subclass it to give MoreColor yellow, I can't expect the > function to suddenly recognise yellow. So I'm not going to use > subclassing in this case, because it can't work. > > But I will use subclassing to reuse values: if I have a function that > expects red/blue/green/yellow, and red/blue/green are already defined > in Color, I'll want to reuse them rather than duplicate them. Hence I > will subclass Color specifically to extend it, not to specialise it. > On the other hand, a function that expects MoreColor should also accept Color, which is a subset. > > The second link quotes: > > "For the most part, extensibility of enums turns out to be a bad > idea. It is confusing that elements of an extension type are > instances of the base type and not vice versa." > > Confusing for who, and why? > > You're going to confuse *somebody* no matter what you do. Either: > > - confuse people who try to subclass enums, and can't; or > > - confuse people who try to subclass enums, and can, but then get > confused by the result. >
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