Changing the repr() of a class?

[Alex Martelli]

Lulu of the Lotus-Eaters wrote:

> Alex Martelli <aleax at aleax.it> wrote previously:
> |class FunkyReprMeta(type):
> |    def __repr__(cls):
> |        return "My wonderful class %s" % cls.__name__
> |class FunkyReprObject:
> |    __metaclass__ = FunkyReprMeta
> |class C(FunkyReprObject):
> |    "whatever you want here"
> 
> I don't really understand what a metaclass buys you here.  I think I
> might be missing something important, since Alex proposes it.

Thanks for the compliment, but you're only missing *the whole point*,
it seems to me.

> However, it appears that one could simply define the __repr__ method in
> the regular class FunkyReprObject, ignore metaclasses altogether, and
> wind up with the same results.

Obviously not: when you define a special method in a class, you control
operation of the *instances* of the class, NOT of the class object itself.

When you want to control operations on the class object itself, you must
then obviously define special methods in the class's class -- i.e., the
metaclass.  That's the main reason for custom metaclasses to exist at all:
the special methods that they define control operations on the classes that
instantiate them (just as a key reason for a class to exist is to define
special methods to control operation on the class's instances).


Alex