[Python-3000] Breakthrough in thinking about ABCs (PEPs 3119 and 3141) (original) (raw)

Joel Bender jjb5 at cornell.edu
Tue May 1 18:16:08 CEST 2007

Phillip J. Eby wrote:

Personally, I still think that the most uniform way of spelling this is overloading isinstance and issubclass; that has the highest likelihood of standardizing the spelling for such inquiries. A big +1 here. This is no different than e.g. operator.mul() being able to do different things depending on the second argument.

n00b here, trying to follow this...

 class X:
     def __mul__(self, y): print self, "mul", y
     def __rmul__(self, y): print self, "rmul", y

Treating isinstance like operator.mul, I could do this (and I would expect that you want to make it a class method)...

 class Y:
     @classmethod
     def __risinstance__(cls, obj): print obj, "is instance of", cls

So issubclass(D, C) would call D.issubclass(C) or C.rissubclass(D) and leave it up to the programmer. The former is "somebody is checking to see if I inherit some functionality" and the latter is "somebody is checking to see if something is a proper derived class of me".

 class A(object):
     @classmethod
     def __rissubclass__(cls, subcls):
         if not object.__rissubclass__(cls, subcls):
             return False
         return subcls.f is not A.f

     def f(self):
         raise RuntimeError, "f must be overridden"

 class B(A):
     def g(self): print "B.g"

 class C(A):
     def f(self): print "C.f"

Now my testing can check issubclass(B, A) and it will fail because B.f hasn't been provided, but issubclass(C, A) passes. I don't have to call B().f() and have it fail, it might be expensive to create a B().

Joel

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