[Python-3000] ABC PEP isinstance issue Was: PEP 31XX: A Type Hierarchy for Numbers (and other algebraic entities)
jimjjewett at gmail.com
Thu Apr 26 00:10:23 CEST 2007
The current ABC proposal is to use isinstance as the test; Jeffrey
Yaskin's numbers PEP highlighted the weakness there with a concrete
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
Short of allowing more __bases__ surgery, we need a function parallel
to isinstance, which at least makes 3rd-party registration possible.
I suspect Phillip will say that we really need to make the ABCs
generic functions... but I disagree; I'm not sure that 3rd-party
adapters should even be allowed by default, but it *should* be simple
to create an ABC that does take them.
def isexample(obj, ABC):
for cls in obj.__class__.__mro__:
result = ABC.meets(obj, cls)
# override this with a dictionary to allow 3rd-party registration
_good_enough = ()
def meets(cls, obj=None, objclass=None):
if objclass is cls: # covers isinstance
if objclass in cls._good_enough: # Nothing is, by default
def assert_sufficient(cls, objclass, adapter):
On 4/25/07, Jeffrey Yasskin <jyasskin at gmail.com> wrote:
> If someone needs to split them later, they can use code like::
> import numbers
> class IntegralDomain(Ring): ...
> numbers.Integral.__bases__ = (IntegralDomain,) + numbers.Integral.__bases__
This only works with old-style classes, which are going away.
>>> class Abstract1(object): pass
>>> class Abstract2(object): pass
>>> Abstract1.__bases__ = (Abstract2,) + Abstract1.__bases__
Traceback (most recent call last):
File "<pyshell#33>", line 1, in <module>
Abstract1.__bases__ = (Abstract2,) + Abstract1.__bases__
TypeError: __bases__ assignment: 'Abstract2' deallocator differs from 'object'
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