Python: Correct way to initialize when superclasses accept different arguments?

If I've got three classes like this:

class BaseClass(object):
    def __init__(self, base_arg, base_arg2=None):
        ...

class MixinClass(object):
    def __init__(self, mixin_arg):
        ...

class ChildClass(BaseClass, MixinClass):
    def __init__(self, base_arg, mixin_arg, base_arg2=None):
        ???

What is the correct way to initialize MixinClass and BaseClass?

It doesn't look like I can use super because the MixinClass and the BaseClass both accept different arguments… And two calls, MixinClass.__init__(...) and BaseClass.__init__(...), will could cause the diamond inheritence problem super is designed to prevent.