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Author Jim Fasarakis-Hilliard
Recipients Jim Fasarakis-Hilliard, georg.brandl, pitrou, rhettinger, serhiy.storchaka
Date 2017-05-11.02:50:15
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Message-id <1494471016.46.0.395643512893.issue30230@psf.upfronthosting.co.za>
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> he said, "I'm not sure if this was placed inside there due to some wild edge-case,"

As a new contributor, I'll always lean on the side of me not seeing something rather than confidently stating that something is definitely wrong. I'm new, the code-base is 20 years old and worked on by a dozen of experienced devs :-)

>  Does a "Not-A-Class" class which is not a subclass of itself (issubclass(C, C) returns False) makes any sense?

I'm not sure. Apart from the performance impact, there's a behavioral discrepancy between isinstance and issubclass as you also stated. 

In isinstance, __instancecheck__ doesn't *always* get a chance to override the behavior. The `if type(inst) == Cls` [1] stops it before it gets a chance. 
In issubclass, __subclasscheck__ does override it:

    class Meta(type):
        def __instancecheck__(self, other):
            print("invoked")
            return False
        def __subclasscheck__(self, other):
            print("invoked")
            return False

    class Cls(metaclass=Meta):
        pass

    isinstance(Cls(), Cls)
    True

    issubclass(Cls, Cls)
    invoked
    False

So, I guess the question might be re-framed to: Is it guaranteed that __instancecheck__ and __subclasscheck__ are *always* called?

If yes: PyObject_IsInstance should be tweaked.
If no:  PyObject_IsSubclass should be tweaked.

p.s Should I maybe move this to python-dev?

[1]: https://github.com/python/cpython/blob/master/Objects/abstract.c#L2338
History
Date User Action Args
2017-05-11 02:50:16Jim Fasarakis-Hilliardsetrecipients: + Jim Fasarakis-Hilliard, georg.brandl, rhettinger, pitrou, serhiy.storchaka
2017-05-11 02:50:16Jim Fasarakis-Hilliardsetmessageid: <1494471016.46.0.395643512893.issue30230@psf.upfronthosting.co.za>
2017-05-11 02:50:16Jim Fasarakis-Hilliardlinkissue30230 messages
2017-05-11 02:50:15Jim Fasarakis-Hilliardcreate