Guido van Rossum wrote:
I've added a comment. Let me know if anything I said is unclear.
On Thu, Jun 12, 2008 at 3:35 AM, Nick Coghlan email@example.com wrote:
The bugtracker seems to be offline atm - I'll reply there once I can get to it again (as well as switching this issue back to being a documentation one).
I don't think we're going to see a major clamor for a value-based delegation mixin in the standard library until people using classic classes for value-based delegation start making serious attempts to port to Py3k (where that approach is no longer available). At the moment, such classes only need to care about the methods they want to fiddle with, leaving everything else to __getattr__ based delegation.
Whether they'll care about this issue of course depends on whether they need overloaded operators and other special delegations to be delegated transparently. We'll have to see how important this is. New-style classes have been around and recommended for a long time -- why haven't people pushed for a proxy class before?
It's only in Python 3 that old style classes are going away fully, so up until now you could at least use a classic class to do the proxying.
I've written my own proxy classes before that look very similar to this, and there are other proxy classes around that do the same (I thought one was by Phillip J Eby but can't find a reference easily). The last one I wrote was to proxy CPython objects from IronPython via Python.NET...
I would prefer it if the proxy class wrapped the return values of inplace operations.
I've pushed as hard as I'm personally willing to for this without convincing anyone else that it's worth doing,
What does *it* refer to? Changing the behavior, or adding a proxy class to the stdlib? I'm -1000 on the former, but only -0 on the latter -- as I wrote in the tracker, I just don't want to see an unproven proxy class (and I don't like the module name).
so I'll start working on a documentation patch for the language reference instead which explicitly splits the special methods into the following categories:
Thanks for doing this, it is needed regardless!
- Method lookup MUST bypass __getattribute__, shadowing the attribute in
the instance dictionary MUST NOT have any ill effects. (All tp_* C-level slots and slots looked up via _PyType_Lookup will fit into this category)
Watch out: I think the term "method lookup" may be confusing here. Certainly when the user writes x.__foo__(), the instance dict *is* consulted. It is only on *implied* lookups (e.g. x[y] or x+y) where the instance dict is bypassed.
- Method lookup MAY bypass __getattribute__, shadowing the attribute in the
instance dictionary MAY have ill effects. (slots such as __enter__ and __exit__ that are looked up via normal attribute lookup in CPython will fit into this category)
Why even have a MAY category? Are you expecting these to become tp_ slots at some point?
- Technically a subcategory of group 1, these are special methods which can
affect the interpreter's behaviour by their mere definition on a type. (The __get__, __set__ and __delete__ descriptor protocol methods fall into this category)
I don't follow why this is relevant. This is a different, AFAIK orthogonal issue, used in many places: *if* an object used in a certain context has a specific attribute, *then* that attribute is used, *otherwise* a default action is taken. Applies to __repr__ just as much. These belong in category 1 if and only if the lookup bypasses the instance dict.