list(ProxyFactory(schema)) does not work on Python 3

[mgedmin]

This code works on Python 2.7:

>>> from zope.interface import Interface
>>> from zope.security.proxy import ProxyFactory
>>> class IFoo(Interface):
...     def x(): 'an method'
...
>>> list(IFoo)
['x']
>>> list(ProxyFactory(IFoo))
['x']

It breaks on Python 3:

>>> list(IFoo)
['x']
>>> list(ProxyFactory(IFoo))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
zope.security.interfaces.ForbiddenAttribute: ('__len__', <InterfaceClass __main__.IFoo>)

AFAICT the reason is that Python's list(an_object) tries to compute len(an_object) as an optimization (to preallocate all the list entries in one go). Python 2 ignores exceptions raised by an_object.__class__.__len__(an_object) when they're an AttributeError subclass and merely skips the optimization. Python 3 does not suppress the exception but instead fails the entire operation.

It's a bit more subtle than that, e.g. if a class doesn't have a __len__, then list() on instances of that class succeeds, but if the class has a __len__ that does raise AttributeError, then list() on instances of that class fails (on Python 3).

I'm not sure if this is a problem in zope.interface or in zope.security. The simplest solution would be to allow __len__ on Interface objects.

[jamadden]

I suspect the fix belongs in zope.security. There have been other zope.interface-on-Python3 fixes necessary there, specifically around iteration and length hints (and also for BTrees) (e.g., zopefoundation/zope.security@6f8de03)