Question

After reading the excellent SO post, I tried crafting a module level metaclass:

def metaclass(future_class_name, future_class_parents, future_class_attrs):
    print "module.__metaclass__"
    future_class_attrs["bar"]="bar"
    return type(future_class_name, future_class_parents, future_class_attrs)

__metaclass__=metaclass


class Foo(object):

    def __init__(self):
        print 'Foo.__init__'

f=Foo()

This doesn’t work (i.e. “module.metaclass” doesn’t get printed) unless I remove the object base class of Foo. How come?

NOTE: I am using Python 2.6.1.

Answer 1

Inheriting from object automatically brings the type metaclass along with it. This overrides your module level __metaclass__ specification.

If the metaclass is specified at the class level, then object won’t override it:

def metaclass(future_class_name, future_class_parents, future_class_attrs):
    print "module.__metaclass__"
    future_class_attrs["bar"]="bar"
    return type(future_class_name, future_class_parents, future_class_attrs)

class Foo(object):
    __metaclass__ = metaclass

    def __init__(self):
        print 'Foo.__init__'

f=Foo()

See http://docs.python.org/reference/datamodel.html?highlight=metaclass#customizing-class-creation