On Thu, Apr 12, 2018 at 11:31 PM, Jacco van Dorp <j.van.d...@deonet.nl> wrote: > 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)
If you want this to be a generic name-binding operation, then no; most objects cannot be used as context managers. You'll get an exception if you try to use "with 1 as x:", for instance. As Nick mentioned, there are context managers that return something other than 'self', and for those, "with expr as name:" has an important meaning that cannot easily be captured with an assignment operator. ChrisA _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
