On Thu, Apr 12, 2018 at 4:38 AM, Ethan Furman <et...@stoneleaf.us> wrote: > On 04/10/2018 10:32 PM, Chris Angelico wrote: > >> Title: Assignment Expressions > > > Thank you, Chris, for doing all this! > > --- > > Personally, I'm likely to only use this feature in `if` and `while` > statements; if the syntax was easier to read inside longer expressions then > I might use this elsewhere -- but as has been noted by others, the > on-the-spot assignment creates asymmetries that further clutter the overall > expression. > > As Paul noted, I don't think parenthesis should be mandatory if the parser > itself does not require them. > > For myself, I prefer the EXPR as NAME variant for two reasons: > > - puts the calculation first, which is what we are used to seeing in > if/while statements; and > - matches already existing expression-level assignments (context managers, > try/except blocks) > > +0.5 from me.
Context managers and except blocks don't do the same thing though, so it's a false parallel. The 'as' keyword occurs in Python's grammar thus: 1) "with EXPR as target:" captures the return value from __enter__ 2) "except EXPR as NAME:" captures the exception value, not the type(s) 3) "import NAME as NAME" loads a module object given by a token (name) and captures that 4) "from NAME import NAME as NAME" captures an attribute of a module Three of them use a name, one allows arbitrary assignment targets. (You can "with spam as ham[0]:" but you can't "except Exception as ham[0]:".) Two of them have no expressions at all, so assignment expressions wouldn't logically be usable. The other two are *dangerous* false parallels, because "with foo as bar:" is semantically different from "with (foo as bar):"; it was so awkward to try to explain that away that I actually forbade any use of "(expr as name)" in the header of a with/except block. The parallel is not nearly as useful as it appears to be on first blush. The parallel with assignment statements is far closer; in fact, many situations will behave identically whether you use the colon or not. > - puts the calculation first, which is what we are used to seeing in > if/while statements; and Not sure what you mean here; if and while statements don't have anything BUT the calculation. A 'for' loop puts the targets before the evaluated expression. > - matches already existing expression-level assignments (context managers, > try/except blocks) They're not expression-level assignments though, or else I'm misunderstanding something here? 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/
