2018-04-12 15:02 GMT+02:00 Nick Coghlan <ncogh...@gmail.com>: > On 12 April 2018 at 22:22, Jacco van Dorp <j.van.d...@deonet.nl> wrote: >> I've looked through PEP 343, contextlib docs ( >> https://docs.python.org/3/library/contextlib.html ), and I couldn't >> find a single case where "with (y := f(x))" would be invalid. > > Consider this custom context manager: > > @contextmanager > def simple_cm(): > yield 42 > > Given that example, the following code: > > with cm := simple_cm() as value: > print(cm.func.__name__, value) > > would print "'simple_cm 42", since the assignment expression would > reference the context manager itself, while the with statement binds > the yielded value. > > Another relevant example would be `contextlib.closing`: that returns > the passed in argument from __enter__, *not* self. > > And that's why earlier versions of PEP 572 (which used the "EXPR as > NAME" spelling) just flat out prohibited top level name binding > expressions in with statements: "with (expr as name):" and "with expr > as name:" were far too different semantically for the only syntactic > difference to be a surrounding set of parentheses. > > Cheers, > Nick.
Makes sense. However, couldn't you prevent that by giving with priority over the binding ? As in "(with simple_cm) as value", where we consider the "as" as binding operator instead of part of the with statement ? Sure, you could commit suicide by parenthesis, but by default it'd do exactly what the "with simple_cm as value" currently does. This does require use of as instead of :=, though. (which was the point I was trying to make, apologies for the confusion) _______________________________________________ Python-ideas mailing list Pythonfirstname.lastname@example.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/