On 26 December 2016 at 04:24, Zahari Dim <zaharid@gmail.com> wrote:
Hi,
The other day I came across a particularly ugly bug. A simplified case goes like:
class X: @property def y(self): return self.nonexisting
hasattr(X(),'y')
This returns False because hasattr calls the property which in turn raises an AttributeError which is used to determine that the property doesn't exist, even if it does. This is arguably unexpected and surprising and can be very difficult to understand if it happens within a large codebase. Given the precedent with generator_stop, which solves a similar problem for StopIteration, I was wondering if it would be possible to have the __get__ method convert the AttributeErrors raised inside it to RuntimeErrors.
The situation with this is a little more complicated because there could be a (possibly strange) where one might want to raise an AttributeError inside __get__.
There are a lot of entirely valid properties that look something like this: @property def attr(self): try: return data_store[lookup_key] except KeyError: raise AttributeError("attr") And unlike StopIteration (where either "return" or "raise StopIteration" could be used), that *is* the way for a property method to indicate "attribute not actually present". This is one of the many cases where IDEs with some form of static structural checking really do make development easier - the "self.nonexisting" would be flagged as non-existent directly in the editor, even before you attempted to run the code. Cheers, Nick. -- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia