[Python-ideas] FW: Map-then-filter in comprehensions

[Mark Mollineaux]

This map-then-filter shortcoming is one of the most common warts I
encounter with python syntax, and I would absolutely love an improvement
here.

I think ChrisA is closest with:

> I'm really not liking the proposed syntax, but maybe there's an alternative.
My first thought on reading this proposal is: "Ah, it's like SQL's 'HAVING'
keyword". The trouble is that SQL can identify its columns, but Python
doesn't have an easy syntax for "the thing you're about to return".
Something like:
>
> [abs(x) for x in numbers having _ > 5]

But I wonder, why not just:

[abs(x) for x in numbers if _ > 5]

Where _ refers to the return expression within a comprehension, and is
interpreted specially in this context?


On Tue, Mar 8, 2016 at 7:20 AM, Chris Angelico <rosuav at gmail.com> wrote:

> On Wed, Mar 9, 2016 at 1:59 AM, Émanuel Barry <vgr255 at live.ca> wrote:
> > I’m going be confused by what sort of name binding it does. A quick
> glance
> > at the keywords list tells me that no currently-existing keyword is “the
> > obvious choice” for this purpose. I don’t think such a small niche
> warrants
> > a new keyword, either.
>
> Maybe a pseudo-keyword would be sufficient - comprehension/genexp
> syntax is pretty solidly specified, so it's less likely to break stuff
> than in general syntax.
>
> I'm really not liking the proposed syntax, but maybe there's an
> alternative. My first thought on reading this proposal is: "Ah, it's
> like SQL's 'HAVING' keyword". The trouble is that SQL can identify its
> columns, but Python doesn't have an easy syntax for "the thing you're
> about to return". Something like:
>
> [abs(x) for x in numbers having _ > 5]
>
> The semantics would be equivalent to:
> def <listcomp>():
>     result = []
>     for x in numbers:
>         _ = abs(x)
>         if _ > 5: result.append(_)
>     return result
>
> A 'having' clause (and, by the way, I'm not too enthused about the
> name, but I'm using it as a placeholder because of SQL's use) would
> have to go after all 'for' and 'if' clauses, and would have access to
> one special name (maybe _ because of its use in interactive mode, or
> maybe something else) which holds the value that would be returned. A
> simple filter-out-the-false-ones could do this:
>
> [regex.match(line) for line in lines having _]
>
> Of course, people could just switch from 'if' to 'having' across the
> board, but this has the same consequences that it does in SQL: now
> *every* row has to be fully processed, only to be discarded at the
> last step. Using 'having' with no underscore would violate common
> sense and good style.
>
> ChrisA
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