[zope2-tracker] [Bug 1197508] [NEW] zope.interface.Interface.implementedBy method can corrupt

[Michael JasonSmith]

Public bug reported:

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

** Affects: zope2
     Importance: Undecided
         Status: New


** Tags: attribute interface typeerror

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https://bugs.launchpad.net/bugs/1197508

Title:
  zope.interface.Interface.implementedBy method can corrupt

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