[Python-3000] ABC PEP isinstance issue Was: PEP 31XX: A Type Hierarchy for Numbers (and other algebraic entities) (original) (raw)

Guido van Rossum guido at python.org
Thu Apr 26 22:25:28 CEST 2007

On 4/25/07, Jim Jewett <jimjjewett at gmail.com> wrote:

The current ABC proposal is to use isinstance as the test; Jeffrey Yaskin's numbers PEP highlighted the weakness there with a concrete example.

If you need to an abstraction less powerful than an existing ABC, you're out of luck; you can't just assert that the existing class is already sufficient, nor can you expect everyone else to use multiple annotations.

I now have a proposal to allow overloading isinstance() and issubclass(), by defining special (class) methods on the second argument. See http://python.org/sf/1708353. Does this need a PEP? The unit test shows that it can be used to support the use case described above:

class ABC(type):

def __instancecheck__(cls, inst):
    """Implement isinstance(inst, cls)."""
    return any(cls.__subclasscheck__(c)
               for c in {type(inst), inst.__class__})

def __subclasscheck__(cls, sub):
    """Implement issubclass(sub, cls)."""
    candidates = cls.__dict__.get("__subclass__", set())
    return any(c in candidates for c in sub.mro())

class Integer(metaclass=ABC):

__subclass__ = {int}

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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