[Python-Dev] [Python-checkins] r86566 - in python/branches/py3k: Doc/glossary.rst Doc/library/inspect.rst Lib/inspect.py Lib/test/test_inspect.py Misc/NEWS Misc/python-wing4.wpr

[Michael Foord]

On 20/11/2010 16:01, Nick Coghlan wrote:
> On Sun, Nov 21, 2010 at 1:29 AM, Michael Foord
> <fuzzyman at voidspace.org.uk>  wrote:
>> Can you give an example of code in a metaclass that may be executed by
>> getattr_static? It's not that I don't believe you I just can't think of an
>> example. Looking up the class and the mro are the only two examples I can
>> think of (klass.__mro__ and instance.__class__ - and they are noted in the
>> docs?) but aren't metaclass specific.
> The description heavily implies that arbitrary Python code won't be
> executed by calling getattr_static, and that isn't necessarily true.
> It's almost certain to be true in the case when the metaclass is type,
> but can't be guaranteed otherwise.

Given the way that member lookups are done by getattr_static I don't 
think any assumptions about the metaclass are made. I'm happy to be 
proven wrong (but would rather fix it than document it as an exception). 
(Actually we assume the metaclass doesn't use __slots__, but only 
because it isn't *possible* for a metaclass to use __slots__.)

> The retrieval of __class__ is a
> normal lookup on the object, so it can trigger all of the things
> getattr_static is trying to avoid (unavoidable if you want to support
> proxy classes at all), and the lookup of __mro__ invokes all of those
> things on the metaclass.

__class__ and mro lookup are noted in the docs as being exceptions. We 
could actually remove the __class__ lookup from the list of exceptions 
by using type(...) instead of obj.__class__.

> I'll see if I'm still of the same opinion after I sleep on it, but my
> first impression of the docs was that they slightly oversold the
> strength of the "doesn't execute arbitrary code" aspect of the new
> function. The existing caveats were all relating to when getattr() and
> getattr_static() might give different answers, while the additional
> caveats I was suggesting related to cases where arbitrary code may
> still be executed.
I'm happy to change the wording to make the promise less strong.

All the best,

Michael

> Cheers,
> Nick.
>


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