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__bases__ Doesn't Work! What's Next?

The following code doesn't work in Python 3.x, but it used to work with old-style classes: class Extender: def extension(self): print('Some work...') class Base: p

Solution 1:

As for me it is impossible. But you can create new class dynamically:

classExtender(object):
    defextension(self):
        print("Some work...")

classBase(object):
    pass

Base = type('Base', (Base, Extender, object), {})
Base().extension()

Solution 2:

It appears that it is possible to dynamically change Base.__bases__ if Base.__base__ is not object. (By dynamically change, I mean in such a way that all pre-existing instances that inherit from Base also get dynamically changed. Otherwise see Mykola Kharechko's solution).

If Base.__base__ is some dummy class TopBase, then assignment to Base.__bases__ seems to work:

class Extender(object):
    def extension(self):
        print("Some work...")

class TopBase(object):
    pass

class Base(TopBase):
    pass

b=Base()
print(Base.__bases__)
# (<class '__main__.TopBase'>,)

Base.__bases__ += (Extender,)
print(Base.__bases__)
# (<class '__main__.TopBase'>, <class '__main__.Extender'>)
Base().extension()
# Some work...
b.extension()
# Some work...

Base.__bases__ = (Extender, TopBase) 
print(Base.__bases__)
# (<class '__main__.Extender'>, <class '__main__.TopBase'>)
Base().extension()
# Some work...
b.extension()
# Some work...

This was tested to work in Python 2 (for new- and old-style classes) and for Python 3. I have no idea why it works while this does not:

classExtender(object):
    defextension(self):
        print("Some work...")

classBase(object):
    pass

Base.__bases__ = (Extender, object)
# TypeError: __bases__ assignment: 'Extender' deallocator differs from 'object'

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