finding abc's
Ian Kelly
ian.g.kelly at gmail.com
Fri Jan 25 14:04:32 EST 2013
On Fri, Jan 25, 2013 at 10:40 AM, lars van gemerden
<lars at rational-it.com> 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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