[Zope-dev] Adapter for class with slots

[Wolfgang Schnerring]

* Shane Hathaway <shane at hathawaymix.org> [2009-12-03 11:44]:
> Wolfgang Schnerring wrote:
>> The minimal reproduction recipe to see the error is this:
>> 
>>     class Slotted(object):
>>         __slots__ = ('__provides__')
>> 
>>     zope.component.provideAdapter(
>>         lambda x: True, (Slotted,), zope.interface.Interface)
>> 
>> Which will raise
>>   File "/home/wosc/.python-eggs/zope.component-3.7.1-py2.6.egg/zope/component/registry.py",
>>   line 419, in _getAdapterRequired
>>       elif not ISpecification.providedBy(r):
>>   TypeError: 'member_descriptor' object is not callable
>
> It looks to me like a possible bug in zope.interface.  Try deleting 
> _zope_interface_coptimizations.so to hopefully expand that traceback.

Thanks for the hint! This enabled me to step through all of it with pdb,
which turns up zope/interface/declarations.py:1249

  def getObjectSpecification(ob):
      provides = getattr(ob, '__provides__', None)
      if provides is not None:
          return provides
      [...]

So it seems that __provides__ is part of the zope.interface mechanics,
and if unset that attribute should not exist -- but when using __slots__
it always exists, namely as a 'member_descriptor' object.

I'll try and see whether I can come up with a way for zope.interface to
do the Right Thing(tm) in this situation.

Wolfgang