[Python-Dev] PEP 572: Assignment Expressions -- intention to accept, near-final draft (original) (raw)

Guido van Rossum guido at python.org
Mon Jul 9 23:43:14 EDT 2018

On Mon, Jul 9, 2018 at 7:40 PM, Glenn Linderman <v+python at g.nevcal.com> wrote:

On 7/9/2018 6:00 PM, Guido van Rossum wrote:

This rule is included to simplify the choice for the user between an assignment statements and an assignment expression -- there is no

"statements" should not be plural in the above line.

Will fix.

syntactic position where both are valid.

An exception to this special case applies when the target name is the same as a loop control variable for a comprehension containing it. This is invalid. This exception exists to rule out edge cases of the above scope rules as illustrated by [i := i+1 for i in range(5)] or [[(j := j) for i in range(5)] for j in range(5)]. Note that this exception also applies to [i := 0 for i, j in stuff], as well as to cases like [i+1 for i in i := stuff]. It is unclear whether exactly what is invalid. Is the use of the target name that is the same as (any of the nested) loop control variable invalid? I think, from discussions, that that is what is meant. But this paragraph could be interpreted as meaning the special case doesn't apply, meaning that the target name would be in a "sublocal" scope.

Really? If it didn't say "this is invalid" I could see that "exception to the special case" might be interpreted as "the special case doesn't apply". But with "This is invalid" explicitly added I don't see how that interpretation could be valid. Is it clearer if I changed that to "Such code is invalid"? Or perhaps I should move "This is invalid" to the end of the paragraph?

The := operator groups more tightly than a comma in all syntactic positions where it is legal, but less tightly than all operators,

If comma is considered an operator, this sentence is inconsistent, would need to be "all other operators". Even if comma is not considered an operator, the sentence would be more clear with "other" added, since ":=" is an operator.

Makes sense.

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