In Python can one implement mixin behavior without using inheritance?

Question

Is there a reasonable way in Python to implement mixin behavior similar to that found in Ruby -- that is, without using inheritance?

class Mixin(object):
    def         


        

Answer 1

This one is based on the way it's done in ruby as explained by Jörg W Mittag. All of the wall of code after if __name__=='__main__' is test/demo code. There's actually only 13 lines of real code to it.

import inspect

def add_mixins(*mixins):
    Dummy = type('Dummy', mixins, {})
    d = {}

    # Now get all the class attributes. Use reversed so that conflicts
    # are resolved with the proper priority. This rules out the possibility
    # of the mixins calling methods from their base classes that get overridden
    # using super but is necessary for the subclass check to fail. If that wasn't a
    # requirement, we would just use Dummy above (or use MI directly and
    # forget all the metaclass stuff).

    for base in reversed(inspect.getmro(Dummy)):
        d.update(base.__dict__)

    # Create the mixin class. This should be equivalent to creating the
    # anonymous class in Ruby.
    Mixin = type('Mixin', (object,), d)

    class WithMixins(type):
        def __new__(meta, classname, bases, classdict):
            # The check below prevents an inheritance cycle from forming which
            # leads to a TypeError when trying to inherit from the resulting
            # class.
            if not any(issubclass(base, Mixin) for base in bases):
                # This should be the the equivalent of setting the superclass 
                # pointers in Ruby.
                bases = (Mixin,) + bases
            return super(WithMixins, meta).__new__(meta, classname, bases,
                                                   classdict)

    return WithMixins 


if __name__ == '__main__':

    class Mixin1(object):
        def b(self): print "b()"
        def c(self): print "c()"

    class Mixin2(object):
        def d(self): print "d()"
        def e(self): print "e()"

    class Mixin3Base(object):
        def f(self): print "f()"

    class Mixin3(Mixin3Base): pass

    class Foo(object):
        __metaclass__ = add_mixins(Mixin1, Mixin2, Mixin3)

        def a(self): print "a()"

    class Bar(Foo):
        def f(self): print "Bar.f()"

    def test_class(cls):
        print "Testing {0}".format(cls.__name__)
        f = cls()
        f.a()
        f.b()
        f.c()
        f.d()
        f.e()
        f.f()
        print (issubclass(cls, Mixin1) or 
               issubclass(cls, Mixin2) or
               issubclass(cls, Mixin3))

    test_class(Foo)
    test_class(Bar)