[ python-Patches-1708353 ] Make isinstance/issubclass overloadable for PEP 3119

Patches item #1708353, was opened at 2007-04-26 16:16
Message generated for change (Comment added) made by gvanrossum
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Category: Core (C code)
Group: Python 3000
>Status: Closed
>Resolution: Accepted
Priority: 5
Private: No
Submitted By: Guido van Rossum (gvanrossum)
Assigned to: Guido van Rossum (gvanrossum)
Summary: Make isinstance/issubclass overloadable for PEP 3119

Initial Comment:
I came up with a fairly simple way to overload isinstance() and issubclass().  The class that is the second argument can define a (class) method named __instancecheck__ or __subclasscheck__ which, if present, will be called *instead* of the normal approach.

The names are different to remind users that the calling convention is the opposite -- isinstance(x, C) maps to C.__instancecheck__(x).


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>Comment By: Guido van Rossum (gvanrossum)
Date: 2007-05-25 18:15

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Checked in. The recursion bug is warded off by a standard recursion check
-- you'll now get an inexplicable RuntimeError exception instead of an
inexplicable SegFault. :-)

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Comment By: Georg Brandl (gbrandl)
Date: 2007-05-12 15:27

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Probably a warning would be good for the non-classmethod case.

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Comment By: Guido van Rossum (gvanrossum)
Date: 2007-05-11 17:40

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FWIW, this can run into unchecked infinite recursion easily:

class C:
    # There's no @classmethod decorator here as there should have been
    def __instancecheck__(self, arg):
        return False

isinstance(42, C)

The reason is that on line 372 in classobject.c there's a call to
PyObject_IsInstance(self, klass).

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Comment By: Guido van Rossum (gvanrossum)
Date: 2007-05-11 10:16

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Version 3 compiles with older C89 compilers like gcc 2.96.
File Added: typechecks.diff

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Comment By: Guido van Rossum (gvanrossum)
Date: 2007-04-26 17:27

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I'll add a description and motivation for this to PEP 3119.

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Comment By: Guido van Rossum (gvanrossum)
Date: 2007-04-26 16:34

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Oops, forget the first attempt; isinstance(Integer(), Integer) would be
False!  Now it's True, and more tests are added.
File Added: typechecks.diff

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