On 20/11/2010 16:01, Nick Coghlan wrote:
On Sun, Nov 21, 2010 at 1:29 AM, Michael Foord
<fuzzy...@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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