[Python-3000] Code freeze?
ncoghlan at gmail.com
Fri Feb 29 15:30:39 CET 2008
Christian Heimes wrote:
> Is this documented somewhere? The docs say "See the
> :meth:`__getattribute__` method below for a way to actually get total
> control over attribute access.".
I just checked, and the restriction is now documented in the development
docs in the section on special methods at . It was backported from
the Py3k docs and isn't part of the docs for 2.5 or earlier versions.
The gist is that special methods *must* be defined directly on a
new-style class in order for the interpreter to reliably access them,
but defining them on a new-style instance *may* still affect the
interpreter's behaviour in some cases (but won't necessarily do so).
In composing this response, I also realised why the new tempfile tests
worked for me before I checked them in: I was testing it on the trunk,
where _TemporaryFileWrapper is a classic class. Special method lookup on
classic classes doesn't include the 'direct-to-type' optimisation, so
those tests work with the tempfile module as written on 2.x.
Unfortunately, proxies like _TemporaryFileWrapper that rely on
__getattr__ to provide special methods simply won't work properly when
implemented as new-style classes - for that, they need to spell out the
overridden special methods the way SpooledTemporaryFile does.
I think we may need a -3 warning that detects situations where "__*__"
attributes are retrieved via a __getattr__ implementation on a classic
class - classes being used that way have a very high chance of breaking
when ported to 3.0 and I can't see any possible way for 2to3 to detect them.
Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia
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