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As discussed at the language summit at PyCon 2011, some code takes some liberties with setting class attributes to callable objects, like CFunctions, that (C)Python does not turn into methods. This is problematic because 1) CPython can't then turn that CFunction into a regular function, and 2) other implementations have to specialcase the builtins to emulate the behaviour.
This patch adds a warning when a non-descriptor, non-class callable attribute is accessed in a context that would have fired the descriptor. It also fixes a bunch of these warnings that show up when running the testsuite. Some of these warnings are obviously (to me) incorrect code -- like the io testsuite, which uses staticmethod around pyio.open but not io.open, or the encoding classes that use an encode function from an extension module as a class attribute. Some of the warnings are debatable, but have an obvious fix (wrap in staticmethod.) Some could be considered false-positives because they actually use (what amounts to) a a bound instance method as a class attribute (for example, re.compile('...').match.)
The patch is still a work in progress, I'm just uploading to start the discussion sooner rather than later.

Don’t forget to remove the comments in codecs that are made obsolete by your changes.

This should not reuse DeprecationWarning but instead introduce a different warning, e.g. CompatibilityWarning.

New version of the patch; instead of triggering at attribute *access*, this just checks when creating types and in type_setattro. It also makes the warning a newly added CompatibilityWarning. Also, instead of warning for all non-descriptors, it now only warns for PyCFunctionObjects that do not have a __self__ set, or of which __self__ is a module. (The reason for the latter being the new-ish behaviour of functions defined in modules in C getting passed something as __self__, defaulting to the module.) I've removed a few staticmethod calls from cases that do not trigger the new warning.
This patch still lacks tests and doc updates, but I'm uploading right now because the sprints apparently lose internet soonish.

Hi Thomas, could you give us a status update? Maybe I can help with tests or docs?

This issue is referred to from #11455.