[Python-ideas] Modern language design survey for "assign and compare" statements

Chris Angelico rosuav at gmail.com
Sat May 19 15:52:59 EDT 2018

On Sun, May 20, 2018 at 4:07 AM, Kirill Balunov <kirillbalunov at gmail.com> wrote:
> [Chris]
>> The bit that you tag on as an afterthought is actually critically
>> important here. You have two options:
>> 1) The 'as' is part of the syntax of the 'if' and 'while' statements; or
>> 2) The 'as' is part of the definition of an expression.
>> The first case would be grammar like this:
>> if_stmt: 'if' test ['as' NAME] ':' suite ('elif' test ':' suite)*
>> ['else' ':' suite]
>> The second is permitting 'as' name-bindings in arbitrary expressions,
>> but then saying "except that they're only allowed in 'if' statements".
>> As you've noted, the first one isn't sufficient. You can't use the
>> restricted syntax for more than a small handful of conditions
>> (including re.match(), but not including anything that might return
>> None and might return other falsey values).
>> So if this is valid:
>>     if (f(x) as y) is not None:
> I think I have a very strong argument "why are not others valid" - Because
> already three months have passed and among 1300+ messages there was not a
> single real example where assignment expression would be convenient or
> useful outside `while` and `if` statements. If you have a counterargument
> (with an example), I would be glad to see.

Okay. Let me put it to you this way, then: Write up a simple and
easy-to-explain set of rules for exactly what is valid and what is
not. Make an actual proposal here. Be sure to demonstrate that the new
construct is useful, with examples (like you're asking me for), and be
sure that you're thorough enough in your definitions that this could
actually be coded. I think you'll find that 1300+ messages aren't
enough to adequately define what you're asking for.

Or, alternatively: Give up on the 'as' syntax, because it was dropped
from the PEP for a reason.


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