class Base:
    def open(self): pass
    def read(self): pass
 
class A(Base):
    def read(self): pass
 
class B(Base):
    def read(self): pass
 
class Mixin:
    def feature(self): pass
 
def add_mixin(item, mixin):
    class _Proxy(Base):
        def __getattribute__(self, name):
            try:
                return getattr(item, name)
            except AttributeError:
                return getattr(mixin, name).__get__(item)
    return _Proxy()
 
for Item in (A, B):
    item = Item()
    item = add_mixin(item, Mixin)
    assert isinstance(item, Base)
    assert item.open.__func__ is Base.open
    assert item.read.__func__ is Item.read
    assert item.feature.__func__ is Mixin.feature

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