On Wed, Apr 18, 2018 at 11:58 PM, Guido van Rossum <gu...@python.org> wrote: > I can't tell from this what the PEP actually says should happen in that > example. When I first saw it I thought "Gaah! What a horrible piece of > code." But it works today, and people's code *will* break if we change its > meaning. > > However we won't have to break that. Suppose the code is (perversely) > > t = range(3) > a = [t for t in t if t] > > If we translate this to > > t = range(3) > def listcomp(t=t): > a = [] > for t in t: > if t: > a.append(t) > return a > a = listcomp() > > Then it will still work. The trick will be to recognize "imported" names > that are also assigned and capture those (as well as other captures as > already described in the PEP).
That can be done. However, this form of importing will have one of two consequences: 1) Referencing an unbound name will scan to outer scopes at run time, changing the semantics of Python name lookups 2) Genexps will eagerly evaluate a lookup if it happens to be the same name as an internal iteration variable. Of the two, #2 is definitely my preference, but it does mean more eager binding. While this won't make a difference in the outermost iterable (since that's *already* eagerly bound), it might make a difference with others: t = range(3) gen = (t for _ in range(1) for t in t if t) t = [4, 5, 6] print(next(gen)) print(next(gen)) Current semantics: UnboundLocalError on first next() call. PEP 572 semantics: Either UnboundLocalError (with current reference implementation) or it yields 1 and 2 (with eager lookups). So either we change things for the outermost iterable, or we change things for everything BUT the outermost iterable. Either way, I'm happy to eliminate the special-casing of the outermost iterable. Yes, it's a change in semantics, but a change that removes special cases is generally better than one that creates them. ChrisA _______________________________________________ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
