[Python-Dev] Informal educator feedback on PEP 572 (was Re: 2018 Python Language Summit coverage, last part)

Nick Coghlan ncoghlan at gmail.com
Sun Jun 24 00:33:59 EDT 2018

On 23 June 2018 at 09:06, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
> Nick Coghlan wrote:
>> x:= f():" implies "x" is already defined as a target somewhere else in
>> the current scope, while "if x := f() given x:" potentially introduces
>> "x" as a new local target
> Noooo..... this is just taking a bad idea and making it
> worse, IMO.
> I'm -1 on any contortions designed to allow comprehensions
> to assign to things in outer scopes. All the proposed use
> cases I've seen for this have not improved readability
> over writing a function that does things the usual way.
> Can we please leave comprehensions as declarative
> constructs? The best tools do just one thing and do
> it well. These proposals seem to be trying to turn
> comprehensions into swiss army knives.

If PEP 572 was proposing the use of regular local scoping for
assignment expressions in comprehensions, such that they could still
be used to avoiding repeating subexpressions within an iteration, but
couldn't be used to export progress data, or to maintain a partial sum
without having to rely on `locals().get("total", 0)` to provide an
initial value, then I wouldn't be feeling obliged to present an
alternative that offers the same state export capabilities in a more
explicit form.

Given that PEP 572 *is* proposing implicit comprehension state export,
though, then I think it's important to make the case that seeing the
proposed semantics as intuitive is only going to be the case for folks
that have used Python 2 style comprehensions extensively - anyone
that's never encountered the old state-leaking behaviour for iteration
variables is going to be surprised when assignment expressions ignore
the existence of the comprehension scope (even though the iteration
variable pays attention to it).


Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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