Using a subclass for __dict__

Demian Brecht demianbrecht at
Thu Feb 13 17:15:02 CET 2014

Hey all,

Using bases in a metaclass, I've been able to easily figure out when
an attribute is being added to an instance of a class. However, what
I'm /actually/ looking to do is to intercept when attributes are being
added to a class (not an instance of). I thought that I'd be able to
do so by passing in a subclass of dict to type.__new__ of the
metaclass's __new__ and implementing a custom __setitem__, but
apparently that's not the case.

If you're curious as to why I'm doing this, I'm trying to clean up and get rid of the
late_bind method there in favour of using the appropriate magic
methods. The intention is to piggyback off of abc's
__abstractmethods__ in order to implement a "looks_like" method that
checks that one class conforms to an unrelated class's interface
rather than using inheritance to enforce interface implementations at
class creation time. This is /only/ intended to be a proof of concept
for demonstration purposes and nothing that I'd ever actually
implement, so no need for flaming the concept :)


class Dict(dict):
    def __setattr__(self, key, value):
        print 'Dict.__setattr__'
        dict.__setattr__(self, key, value)

    def __setitem__(self, key, value):
        print 'Dict.__setitem__'
        dict.__setitem__(self, key, value)

class Bar(object):
    def __setattr__(self, key, value):
        print 'Bar.__setattr__'
        object.__setattr__(self, key, value)

class metafoo(type):
    def __new__(mcls, name, bases, dct):
        return type.__new__(mcls, name, (Bar,), Dict(dct))

class Foo(object):
    __metaclass__ = metafoo

>>> import test
>>> f = test.Foo()
>>> = 'bar'
Bar.__setattr__ # expected
>>> = 'bar'
# I'm expecting Dict.__setattr__ here, but... *crickets*

Am I missing something here, or do I just have to live with what I
currently have in my gist?


Demian Brecht

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