[Python-ideas] PEP 572 version 2: Statement-Local Name Bindings

[Brendan Barnwell]

On 2018-03-02 12:20, Chris Angelico wrote:
> On Sat, Mar 3, 2018 at 7:04 AM, Eric Fahlgren <ericfahlgren at gmail.com> wrote:
>>
>> On Fri, Mar 2, 2018 at 10:27 AM, Jelle Zijlstra <jelle.zijlstra at gmail.com>
>> wrote:
>>>
>>>
>>> I wonder if we could have a more limited change to the language that would
>>> allow only the as/while use cases. Specifically, that means we could do:
>>>
>>> while do_something() as x:
>>>     print(x)
>>
>>
>> The "while" case is the only part of the PEP that has any traction with me.
>> It doesn't add any keywords, scope can be identical to "with" and it cleans
>> up a code pattern that is very common.
>
> How often do you have a loop like this where you actually want to
> capture the exact condition? I can think of two: regular expressions
> (match object or None), and socket read (returns empty string on EOF).
> This simplified form is ONLY of value in that sort of situation; as
> soon as you want to add a condition around it, this stops working (you
> can't say "while do_something() is not _sentinel as x:" because all
> you'll get is True). And if you are looking for one specific return
> value as your termination signal, you can write "for x in
> iter(do_something, None):".

	But you could have "while (do_something() as x) is not _sentinel".  Not 
sure how proponents and opponents would react to that.  Limiting the 
SLNB to the beginning of block-level statements seems perverse in a way, 
but also might cut down on gratuitous overuse mixed into all kinds of 
weird positions in statements.

-- 
Brendan Barnwell
"Do not follow where the path may lead.  Go, instead, where there is no 
path, and leave a trail."
    --author unknown