Today I have come across a surprising definition of a metaclass in Python here, with the metaclass definition effectively inlined. The relevant part is

class Plugin(object):
    class __metaclass__(type):
        def __init__(cls, name, bases, dict):
            type.__init__(name, bases, dict)
            registry.append((name, cls))

When does it make sense to use such an inline definition?

Further Arguments:

An argument one way would be that the created metaclass is not reusable elsewhere using this technique. A counter argument is that a common pattern in using metaclasses is defining a metaclass and using it in one class and then inhertiting from that. For example, in a conservative metaclass the definition

class DeclarativeMeta(type):
    def __new__(meta, class_name, bases, new_attrs):
        cls = type.__new__(meta, class_name, bases, new_attrs)
        cls.__classinit__.im_func(cls, new_attrs)
        return cls
class Declarative(object):
    __metaclass__ = DeclarativeMeta
    def __classinit__(cls, new_attrs): pass

could have been written as

class Declarative(object):  #code not tested!
    class __metaclass__(type):
        def __new__(meta, class_name, bases, new_attrs):
            cls = type.__new__(meta, class_name, bases, new_attrs)
            cls.__classinit__.im_func(cls, new_attrs)
            return cls
    def __classinit__(cls, new_attrs): pass

Any other considerations?

python metaclass
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