gistfile1.py

import six

class ModelBase(type):
    def __new__(meta, name, bases, dct):
        print("ModelBase.__new__(%s)" % name)
        result = super(ModelBase, meta).__new__(meta, name, bases, dct)
        print("/ModelBase.__new__(%s)" % name)
        return result

    def __init__(cls, name, bases, dct):
        print("ModelBase.__init__(%s)" % name)
        super(ModelBase, cls).__init__(name, bases, dct)
        print("/ModelBase.__init__(%s)" % name)

class Model(six.with_metaclass(ModelBase, object)):
    pass

print("\n")

class PageBase(ModelBase):
    def __new__(meta, name, bases, dct):
        print("PageBase.__new__(%s)" % name)
        result = super(PageBase, meta).__new__(meta, name, bases, dct)
        print("/PageBase.__new__(%s)" % name)
        return result

    def __init__(cls, name, bases, dct):
        print("PageBase.__init__(%s)" % name)
        super(PageBase, cls).__init__(name, bases, dct)
        print("/PageBase.__init__(%s)" % name)

class Page(six.with_metaclass(PageBase, Model)):
    pass

print("\n")

class Mixin(Model):
    pass

print("\n")

class BlogPost(Mixin, Page):
    pass

print("\n")

class DonationsPage(Page, Mixin):
    pass


"""
On Python 2.7, putting Mixin ahead of Page in BlogPost's superclass list
results in a call chain of ModelBase -> PageBase -> ModelBase -
i.e. ModelBase.__new__ is called twice:

ModelBase.__new__(BlogPost)
PageBase.__new__(BlogPost)
ModelBase.__new__(BlogPost)
/ModelBase.__new__(BlogPost)
/PageBase.__new__(BlogPost)
/ModelBase.__new__(BlogPost)
PageBase.__init__(BlogPost)
ModelBase.__init__(BlogPost)
/ModelBase.__init__(BlogPost)
/PageBase.__init__(BlogPost)

On Python 3, the call chain is PageBase -> ModelBase:

PageBase.__new__(BlogPost)
ModelBase.__new__(BlogPost)
/ModelBase.__new__(BlogPost)
/PageBase.__new__(BlogPost)
PageBase.__init__(BlogPost)
ModelBase.__init__(BlogPost)
/ModelBase.__init__(BlogPost)
/PageBase.__init__(BlogPost)
"""