On Sun, 06 Nov 2005 12:23:02 -0500, Christopher Subich <[EMAIL PROTECTED]> wrote:
>Bengt Richter wrote: >> On Fri, 04 Nov 2005 10:28:52 -0500, Christopher Subich <[EMAIL PROTECTED]> >> wrote: > >>>is very much within the language specification. Indeed, the language >>>specification dictates that an instance variable b.a is created if one >>>didn't exist before; this is true no matter if type(b.a) == int, or if >>>b.a is some esoteric mutable object that just happens to define >>>__iadd__(self,type(other) == int). >> >> But if it is an esoteric descriptor (or even a simple property, which is >> a descriptor), the behaviour will depend on the descriptor, and an instance >> variable can be created or not, as desired, along with any side effect you >> like. > >Right, and that's also language-specification. Voodoo, yes, but >language specification nonetheless. :) I guess http://docs.python.org/ref/augassign.html is the spec. I notice its example at the end uses an old-style class, so maybe it's understandable that when it talks about getattr/setattr, it doesn't mention the possible role of descriptors, nor narrow the meaning of "evaluate once" for a.x to exclude type(a).x in the setattr phase of execution. I.e., if x is a descriptor, "evaluate" apparently means only type(a).x.__get__(a, type(a)) since that is semantically getting the value behind x, and so both of the ".x"s in type(a).x.__set__(a, type(a).x.__get__(a, type(a)).__add__(1)) # (or __iadd__ if defined, I think ;-) don't count as "evaluation" of the "target" x, even though it means that a.x got evaluated twice (via getattr and setattr, to get the same descriptor object (which was used two different ways)). I think the normal, non-descriptor case still results in (optimized) probes for type(a).x.__get__ and type(a).x.__set__ before using a.__dict__['x']. ISTM also that it's not clear that defining __iadd__ does _not_ prevent the setattr phase from going ahead. I.e., a successful __iadd__ in-place mutation does not happen "instead" of the setattr. Regards, Bengt Richter -- http://mail.python.org/mailman/listinfo/python-list
