On 08/24/2010 02:31 PM, Benjamin Peterson wrote:
2010/8/24 Hrvoje Niksichrvoje.firstname.lastname@example.org:
The __length_hint__ lookup expects either no exception or AttributeError, and will propagate others. I'm not sure if this is a bug. On the one hand, throwing anything except AttributeError from __getattr__ is bad style (which is why we fixed the bug by deriving our business exception from AttributeError), but the __length_hint__ check is supposed to be an internal optimization completely invisible to the caller of list().
__length_hint__ is internal and undocumented, so it can do whatever it wants.
Of course, but that's beside the point. In this case __length_hint__ was neither implemented in the class, nor were we aware of its existence, and the code still broke (as in the example in previous mail). The point I'm making is that:
a) a "business" case of throwing anything other than AttributeError from __getattr__ and friends is almost certainly a bug waiting to happen, and
b) making the proposed change is bound to break real, production code.
I still agree with the proposed change, but I wanted to also point out that it will cause breakage and illustrate it with a similar real-world example that occurred during migration to python 2.6.