[zope2-tracker] [Bug 374810] Re: [DM] Simplify __bobo_traverse__ implementations by centralizing ZPublisher responsibilities.

[Christian Zagrodnick]

** Description changed:

  2. Introduction
  
  Zope2 allows objects to customize traversal by defining the method
  __bobo_traverse__.
  
  __bobo_traverse__ is used in two quite different contexts: ZPublisher
  traversal and OFS traversal.
  
  ZPublisher requires that its traversal steps return
  webdav.NullResource.NullResource instances and must not use acquisition
  under some (quite difficult to understand and to check) conditions. This
  requirement poses a heavy burden on __bobo_traverse__ authors and leads
  to very volatile implementations. In Archetypes, for example, three
  rounds of bug fixing were necessary before a rather simple
  __bobo_traverse__ (it checks whether an object can be found in the
  session) finally adhered to the ZPublisher requirements. The special
  ZPublisher logic (to not use acquisition and to return NullResource in
  some cases) should be in ZPublisher alone and not be duplicated in all
  kinds of __bobo_traverse__ implementations.
  
  3. Feature
  
  Give __bobo_traverse__ a way to concentrate on special cases only and
  defer to standard traversal in all other cases.
  
  To this end, ZPublisher defines an exception UseTraversalDefault. If a
  call to __bobo_traverse__ raises this exception, the normal traversal
  (including all special casing in case of ZPublisher) is performed.
  
  The implementation requires modifications in
  ZPublisher.BaseRequest.DefaultPublishTraverse.publishTraverse and
  OFS.Traversable.unrestrictedTraverse.
  
  I define UseTraversalDefault in ZPublisher.exceptions. This introduces a
  dependence of OFS.Traversable from ZPublisher.
  
  4. Example Use Cases
  
  The Archetypes __bobo_traverse__ can simply look whether the object is
  found in the session, and return it in this case. Otherwise, it raises
  UseTraversalDefault. The complete difficult logic to guess whether
  __bobo_traverse__ was called from ZPublisher and to determine whether or
  not to allow acquisition and to return NullResource in some failure
  cases goes away.
  
  Other __bobo_traverse__ implementations gains in simplicity and
  robustness in a similar way.
  
  5. Notes
  
-     * Probably other traversal hooks, too, should be allowed to
-           o use this traversal default.
+     * Probably other traversal hooks, too, should be allowed to use this
+ traversal default.

** Changed in: zope2
     Assignee: (unassigned) => Christian Zagrodnick (zagy)

-- 
[DM] Simplify __bobo_traverse__ implementations by centralizing ZPublisher responsibilities. 
https://bugs.launchpad.net/bugs/374810
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