[Python-ideas] Generator syntax hooks?

[Nick Coghlan]

On 10 August 2017 at 01:49, Soni L. <fakedme+py at gmail.com> wrote:
> On 2017-08-09 11:54 AM, Nick Coghlan wrote:
>> Right, I was separating the original request to make "{x for x in
>> integers if 1000 <= x < 1000000}" work into the concrete proposal to
>> make exactly *that* syntax work (which I don't think is feasible), and
>> the slightly more general notion of offering a more math-like syntax
>> that allows finite sets to be built from infinite iterators by
>> defining a termination condition in addition to a filter condition.
>
> Ok. A concrete proposal would give a read-only 'filter' argument to the
> iterator somehow, which represents some form of simplified AST of the
> condition.
>
> So e.g. {x for x in integers if (lambda v: 1000 <= v < 1000000)(x)} would
> never complete, but {x for x in integers if 1000 <= x < 1000000} would. (But
> perhaps lambda objects should include an AST attribute... Having it for
> normal functions would introduce too much overhead tho, and then it would no
> longer be a simplified AST, but rather a complete python AST, which we don't
> want.)

There have been a variety of different "thunking" proposals over the
years, but they've all foundered on the question of what the
*primitive* quoted form should look like, and how the thunks should
subsequently be executed.

For cases like this, where integration with Python's name resolution
mechanism isn't actually required, folks have ended up just using
strings, where the only downside is the fact that syntax highlighters
and other static analysers don't know that the contents are supposed
to be valid Python code. In a case like this, that might look like:

    {x for x in integers.build_set("1000 <= x < 1000000")}

As with regexes, the cost of dynamically parsing such strings can then
be amortised at runtime through the use of an appropriate caching
strategy.

Cheers,
Nick.

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