finding abc's

Ian Kelly ian.g.kelly at
Fri Jan 25 20:04:32 CET 2013

On Fri, Jan 25, 2013 at 10:40 AM, lars van gemerden
<lars at> wrote:
> Hi all,
> i was writing a function to determine the common base class of a number classes:
> and ran common_base(int, float), hoping to get numbers.Number.
> this did not work because abstract base classes are not always in the mro() of classes.
> My question is: is there a way to obtain the abc's of a class or otherwise a way to make the function above take abc's into account (maybe via a predefined function)?

If the abstract base class's module has not been imported, it may not
even be loaded into memory, even though it is technically considered a
superclass.  Consider this:

Python 2.7.1 (r271:86832, Nov 27 2010, 18:30:46) [MSC v.1500 32 bit
(Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> def common_base(classes):
...     common = set()
...     for cls in object.__subclasses__():
...         if all(issubclass(c, cls) for c in classes):
...             common.add(cls)
...     return common
>>> common_base([int, float])
set([<class '_abcoll.Hashable'>])
>>> import numbers
>>> common_base([int, float])
set([<class 'numbers.Number'>, <class '_abcoll.Hashable'>])

If you're okay with that, then the approach above might work.

>     while len(common) > 1:
>         cls1 = common.pop()
>         cls2 = common.pop()
>         if issubclass(cls1, cls2):
>             common.add(cls1)
>         elif issubclass(cls2, cls1):
>             common.add(cls2)

There is a flaw with your set reduction code here.  If neither class
is a subclass of the other, then both will be removed.  There may not
actually be a single closest common base class, however.  What would
you expect the function to return in the following situation?

class A(object): pass
class B(object): pass
class C(A, B): pass
class D(A, B): pass

print common_base([C, D])

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