Get class name in attribute definition

Question

I need to make a mixin that knows the name of the class who is using it. Sort of like:

class FooMixin(...):
    bar = self.__class__

Except that self is not defined at attribute definition time. Is there a clean way to achieve this so it's transparent for the class who inherits the mixin?

Answer 1

At the time of definition of your mixin, nobody knows in which classes your mixin is used. You can only get the name dynamically in class methods by using self.__class__.__name__ :

class FooMixin(object):
    def some_method(self):
        print "I'm in class %s" % self.__class__.__name__

class Main(FooMixin):
    pass

instance = Main()
instance.some_method() # "I'm in class Main"

Answer 2

Daniel's answer gives the reason why this is not possible in the declarative way you seem to like it - nobody knows at Mixin's definition-time where and when it will be used.

However, if you don't care about the time, but want the syntax , meaning you want to access bar defined as property in Mixin, and return self. class , this should work:

class classproperty(object):

    def __get__(self, instance, clazz):
        return clazz

class Mixin(object):

    bar = classproperty()


class Foo(Mixin):
    pass


print Foo().bar

Answer 3

First off, no special action is needed to know the name of a class:

class MyMixin(object):
    def frob(self):
        print "frobbing a", self.__class__.__name__

class Foo(MyMixin): pass
class Bar(MyMixin): pass

>>> Foo().frob()
frobbing a Foo
>>> Bar().frob() 
frobbing a Bar

similarly, no special action is needed to discover subclasses:

>>> MyMixin.__subclasses__()
[__main__.Foo, __main__.Bar]

If these aren't what you need, because you want to take action when your base class is subclassed, you need a metaclass! :

class MyMixinMeta(type):
    def __init__(cls, name, bases, attrs):
        if bases != (object,):
            print name, cls, "is a subclass of", bases

class MyMixin(object):
    __metaclass__ = MyMixinMeta
    def frob(self):
        print "frobbing a", self.__class__.__name__

>>> class Foo(MyMixin): pass
Foo <class '__main__.Foo'> is a subclass of (<class '__main__.MyMixin'>,)
>>> class Bar(MyMixin): pass
Bar <class '__main__.Bar'> is a subclass of (<class '__main__.MyMixin'>,)