Python metaclass and the object base class

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.

score 3 · Accepted Answer · edited Oct 31 '11 at 19:35

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

The specification says no such thing. What it does say is that if there is a baseclass, the metaclass of the baseclass will be used, regardless of whether or not the baseclass specifies a metaclass. — Marcin, May 30 '12 at 15:11

score 2 · Answer 2 · edited Aug 28 '13 at 16:00

The specification specifies the order in which Python will look for a metaclass:

The appropriate metaclass is determined by the following precedence rules:

If dict['__metaclass__'] exists, it is used.

Otherwise, if there is at least one base class, its metaclass is used (this looks for a __class__ attribute first and if not found, uses its type).

Otherwise, if a global variable named __metaclass__ exists, it is used.

Otherwise, the old-style, classic metaclass (types.ClassType) is used.

You will see from the above that having a base class at all (whatever the base class is, even if it does not ultimately inherit from object) pre-empts the module-level __metaclass__.

Python metaclass and the object base class

2 Answers2

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