Acquisition: attribute assignment not faithful

[Dieter Maurer]

Usually, when you assign a value to an attribute, you would expect to retrieve the same value later from this attribute. This is not the case for "ExtensionClass" instances, as they may call the attributes "__of__" method (if any) and return its result -- a well documented feature.

The idiom "obj.__dict__[attr]" can be used to prevent "ExtensionClass" from interfering. Nevertheless, attribute assignment is not faithful for acquisition wrappers, i.e. in some cases

   >>> wrapper.attr = value
   >>> wrapper.__dict__['attr'] is value
  False

The reason for this: acquisition wrapper attribute assignment explicitely removes the acquisition context before storing the
value on the wrapped object.

Obviously, the implementer thought this a feature -- but forgot to document it.

The feature leads to highly surprising behaviour: usually acquisition wrappers mask themselves very successfully as the wrapped objects. Rarely, developers are concious whether they work with a wrapper or the base object. Now, while acquisition wrapped attributes values can faithfully be stored on the base object (e.g. in the __init__ method), they are no longer faithfully stored when the same object is wrapped. Yesterday, I spent several hours to analyse a problem caused by this behaviour.

As I expect that the feature cannot be removed (for backward compatibility reasons), I suggest to document it clearly.

Here an example for the unfaithful attribute assignment:

>>> from Acquisition import Implicit
>>> class C(Implicit):
... pass
...
>>> c0 = C()
>>> c1 = C().__of__(c0)
>>> c2 = C().__of__(c0)
>>> c1.x = c2
>>> c1.__dict__['x'] is c2
False

[Hanno Schlichting (hannosch)]

Patches for the documentation are welcome.

[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The zope2 project on Launchpad has been archived at the request of the Zope developers (see https://answers.launchpad.net/launchpad/+question/683589 and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.