[Python-ideas] Generator syntax hooks?

[Chris Barker]

On Thu, Aug 10, 2017 at 1:53 PM, Spencer Brown <spencerb21 at live.com> wrote:

> The logical solution to me is to allow any order of while and if, and
> follow the same 'rule' as multiple for loops - just nest/test those in that
> order.
>

Actually, I think it would be better to only allow one order, and have the
"while" always teeted first -- which may mean it should be placed first for
clarity.


> Then you can have whatever priority you need. One question though is how
> this should handle multiple loops - break all of them, or just the current
> one?
>

just the current one, just like a "break", or for that matter, a nested
while...

-CHB





> - Spencer Brown
>
> On 11 Aug 2017, at 6:27 am, Chris Barker <chris.barker at noaa.gov> wrote:
>
> On Thu, Aug 10, 2017 at 8:39 AM, Paul Moore <p.f.moore at gmail.com> wrote:
>
>
>>  Also, there's a potential issue
>> here - consider
>>
>>     [expr for var in even_numbers() if is_odd(var) while var < 100]
>>
>> This is an infinite loop, even though it has a finite termination
>> condition (var < 100), because we only test the termination condition
>> if var is odd, which it never will be.
>>
>
> why is the termination only tested if teh if clause is True? Could then
> not be processed in parallel? or the while first....
>
> so maybe better to do:
>
> [expr for var in even_numbers() while var < 100 if is_odd(var)]
>
> Maybe it's just me, but I would certainly expect the while to have
> precedence.
>
> I guess I think of it like this:
>
> "if" is providing a filtering mechanism
>
> "while" is providing a termination mechanism
>
>  -- is there a use case anyone can think of when they would want the while
> to be applied to the list AFTER filtering?
>
> Obviously, this is a contrived example. And certainly "don't do that,
>> then" is a valid response. But my instinct is that people are going to
>> get this wrong - *especially* in a maintenance environment.
>
>
> sure, but would there be an issue if teh while were given precedence?
>
> Overall, I agree with Steven's point. It seems pretty obvious what the
>> intention is, and while it's probably possible to construct examples
>> that are somewhat unclear,
>>
>> 1. The mechanical rule gives an explicit meaning
>> 2. People shouldn't be writing such complex comprehensions, so if the
>> rule doesn't give what they expect, they can always rewrite the code
>> with an explicit (and clearer) loop.
>>
>
> me too -- a direct translation to a for loop isn't necessary to understand
> how it works.
>
> -CHB
>
>
> --
>
> Christopher Barker, Ph.D.
> Oceanographer
>
> Emergency Response Division
> NOAA/NOS/OR&R            (206) 526-6959   voice
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>
> Chris.Barker at noaa.gov
>
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-- 

Christopher Barker, Ph.D.
Oceanographer

Emergency Response Division
NOAA/NOS/OR&R            (206) 526-6959   voice
7600 Sand Point Way NE   (206) 526-6329   fax
Seattle, WA  98115       (206) 526-6317   main reception

Chris.Barker at noaa.gov
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