[Zope3-dev] verifyObject failing for all field types

[Garrett Smith]

A recent change is causing fields to fail verifyObject. E.g. adding the
following test to test_date.py:

    def testInterface(self):
        from zope.interface.verify import verifyObject
        from zope.schema.interfaces import IDate
        verifyObject(IDate, self._Field_Factory())

results in:

BrokenMethodImplementation: The implementation of constraint violates
its contract
        because Not a method.

'constraint' is None, and therefore not callable, which is causing the
failure.

Should the check on callable pass if the attribute is None? Or do we
need to add a no-op callable for the 'constraint' default value for
Field?

 --Garrett