[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 22:36:46 CEST 2007

On 4/26/07, Guido van Rossum <guido at python.org> wrote:

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}

That example has a bug. Try this instead:

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()) | {cls}
    return any(c in candidates for c in sub.mro())

class Integer(metaclass=ABC):

__subclass__ = {int}

To complete the example:

isinstance(42, Integer) and issubclass(int, Integer) now both return True, while isinstance(3.14, Integer) and issubclass(float, Integer) return False. Of course, isinstance(Integer(), Integer) and issubclass(Integer, Integer) are both True. If you subclass Integer, int won't be considered a subclass of it!

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

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