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I think it was David Beazley who characterized metaclasses as "infecting" an inheritance tree. If `class A` has a metaclass, then all subclasses of `A` will in

• jamesdutc 13y ago
  This "infectious behaviour" (though perhaps unfairly characterised) leads to the clearest purpose of metaclasses. Metaclasses (and build_class) is a mechanism by which to enforce a constraint from a base type to a derived type. (Note that, in practice, there is some trickiness around metaclasses on derived classes: http://seriously.dontusethiscode.com/2013/04/18/derived-metaclasses.html http://seriously.dontusethiscode.com/2013/04/18/derived-meta...) It's trivial to enforce a constraint from a derived type to a base type. e.g., # base.py class Base: def spam(self): pass # derived.py class Derived(Base): assert hasattr(Base, 'spam') # or abc, &c. def ham(self): return self.spam But how can we enforce a constraint in the other direction? (e.g., abc.ABCMeta) # base.py from functools import wraps class metaclass(type): def __new__(m, n, b, d): assert 'spam' in d # must implement, not just inherit # can even enforce behaviour via wrapping spam = d['spam'] @wraps(spam) def wrap(*args, **kwargs): print('wrap({}, {})'.format(args, kwargs)) return spam(*args, **kwargs) d['spam'] = wrap return type.__new__(m, n, b, d) class Base(metaclass=metaclass): def spam(self): pass # derived.py class Derived(Base): def spam(self): pass