zope.interface.Interface.implementedBy method can corrupt

[Michael JasonSmith]

Incorrectly using zope.interface.Interface.implementedBy on an object, rather than a class, can corrupt the object and make it unusable. Any time an attribute of the object is accessed, after using implementedBy, the following error is raised:

  File "…/zope/interface/declarations.py", line 338, in implementedByFallback
    raise TypeError("ImplementedBy called for non-factory", cls)

  TypeError: (TypeError('ImplementedBy called for non-factory', <Folder at >),
              <built-in function implementedBy>, (<Folder at >,))

It does not seem to matter which attribute is accessed, or how (getattr causes the same error as o.attr).

It appears that the root cause of the problem is a side-effect of zope.interface.declarations.implementedByFallback, which is called by implementedBy:

         cls.__implemented__ = spec

         https://github.com/zopefoundation/zope.interface/blob/master/src/zope/interface/declarations.py#L199

At the very least, implementedBy should not have a side-effect. Ever.

This side-effect turns a simple mistake into a major problem:
  http://groupserver.org/r/post/5zxOdqzn3dUyweYCAzfahP

I note that others have had similar issues:
  http://blog.fourdigits.nl/how-to-break-your-plone-site-with-implementedby

[Colin Watson (cjwatson)]

The zope2 project on Launchpad has been archived at the request of the Zope developers (see https://answers.launchpad.net/launchpad/+question/683589 and https://answers.launchpad.net/launchpad/+question/685285). If this bug is still relevant, please refile it at https://github.com/zopefoundation/zope2.