Message 307013 - Python tracker (original) (raw)
To clarify, the outermost iterator is marked here:
[func(x, y) for x in <outermost iterator> for y in <something else>]and it is evaluated before the comprehension proper is started. This is just part of how the language works (it's a similar rule as "in an assignment the RHS is evaluated before it is assigned to the LHS").
I see no benefit in banning yield in .
On Sat, Nov 25, 2017 at 10:22 PM, Guido van Rossum <report@bugs.python.org> wrote:
Guido van Rossum <guido@python.org> added the comment:
No.
On Nov 25, 2017 18:01, "Nick Coghlan" <report@bugs.python.org> wrote:
Nick Coghlan <ncoghlan@gmail.com> added the comment:
Serhiy's PR now implements the "Prohibit yield & yield from in generator expressions and comprehensions" approach discussed on python-dev (currently as a hard SyntaxError, but it could be amended to be a warning instead without too much difficulty).
The PR does highlight an interesting subtlety though: the easiest way to implement this still allows yield expressions in the outermost iterator, since that gets compiled in a different scope from the rest of the comprehension body (it's evaluated eagerly and passed in to the implicit nested function).
I'm thinking we probably don't want to expose that detail to end users, and will instead want to include a second check that prohibits yield expressions in the outermost iterator as well. However, I'm not entirely sure how we could implement such a check, short of adding a new "yield expression prohibited" counter in the AST generation and/or symbol analysis pass.
Python tracker <report@bugs.python.org> <https://bugs.python.org/issue10544>
Python tracker <report@bugs.python.org> <https://bugs.python.org/issue10544>