[Python-ideas] A PEP on introducing variables on 'if' and 'while'
steve at pearwood.info
Sun Jun 10 09:19:58 EDT 2018
On Sun, Jun 10, 2018 at 08:08:13AM -0400, Juancarlo Añez wrote:
> > while (condition := expression) as flag:
> > ...
> Ah! Are the parenthesis necessary there?
It's your PEP, you tell us.
> > Accepting "while/if as name" would remove much (but not all) of the
> > motivation for assignment expressions, while accepting assignment
> > expressions would make a dedicated while/if as name syntax unnecessary.
> Most of the arguments in favor of ':=' have been through examples of
> generators in which the introduced name is used within, with the if/while
> case often forgotten.
Indeed. I think that the generator/comprehension use-cases for
assignment expressions are not the most compelling. They're important,
but at the point you need to use assignment inside a comprehension,
there's a good argument that it *may* be time to refactor to a loop. I
think the while/if examples are even more important.
> There can be cases in which combining both syntaxes is useful:
> x = None
> while compute(x := v for v in next_series_value(x)) as comp:
> x = comp
But if you have assignment expressions, you don't need "as".
x = None
while comp := compute(x := v for v in next_series_value(x)):
x = comp
> Finding a real-world example of something like the above synthetic example
> would be in favor of the orthogonality.
As the PEP author, that's your job.
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