finding abc's

lars van gemerden lars at rational-it.com
Fri Jan 25 21:08:12 CET 2013


On Friday, January 25, 2013 8:08:18 PM UTC+1, Peter Otten wrote:
> lars van gemerden wrote:
> 
> 
> 
> > Hi all,
> 
> > 
> 
> > i was writing a function to determine the common base class of a number
> 
> > classes:
> 
> > 
> 
> > def common_base(classes):
> 
> >     if not len(classes):
> 
> >         return None
> 
> >     common = set(classes.pop().mro())
> 
> >     for cls in classes:
> 
> >         common.intersection_update(cls.mro())
> 
> >     while len(common) > 1:
> 
> >         cls1 = common.pop()
> 
> >         cls2 = common.pop()
> 
> >         if issubclass(cls1, cls2):
> 
> >             common.add(cls1)
> 
> >         elif issubclass(cls2, cls1):
> 
> >             common.add(cls2)
> 
> >     return common.pop()
> 
> > 
> 
> > 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)?
> 
> 
> 
> The abstract base classes may run arbitrary code to determine the subclass 
> 
> relationship:
> 
>  
> 
> >>> from abc import ABCMeta
> 
> >>> import random
> 
> >>> class Maybe(metaclass=ABCMeta):
> 
> ...     @classmethod
> 
> ...     def __subclasshook__(cls, C):
> 
> ...             print("processing", C)
> 
> ...             return random.choice((False, True))
> 
> ... 
> 
> >>> isinstance(1.1, Maybe)
> 
> processing <class 'float'>
> 
> True
> 
> >>> isinstance(1.1, Maybe)
> 
> True
> 
> >>> isinstance(1, Maybe)
> 
> processing <class 'int'>
> 
> False
> 
> >>> issubclass(float, Maybe)
> 
> True
> 
> 
> 
> You'd have to check every pair of classes explicitly and might still miss 
> 
> (for example) numbers.Number as the module may not have been imported. 
> 
> 
> 
> I think you are out of luck.

Thank you, interesting example. Added confirmation that trying to get the abc's is a bad idea. 



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