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

[Allan Clark]

On 8 March 2016 at 16:02, Paul Moore <p.f.moore at gmail.com> wrote:

> On 8 March 2016 at 14:17, Allan Clark <allan.clark at gmail.com> wrote:
> > tl;dr What is support like for adding an 'as' clause to comprehension
> > syntax? In order to allow map-then-filter, it might look like something
> > this:
> >
> >     [y for x in numbers if abs(x) as y > 5]
> >
> > I wish to propose an extension to Python comprehension syntax in an
> attempt
> > to make it applicable in more areas.
>
> [snip, fair comment]
>


> So what seems to me to be missing from your proposal is an explanation
> of how extending the comprehension syntax is an improvement over not
> using a comprehension at all.
>

Yeah that's a pretty fair observation.


> You suggest
>
>     [y for x in numbers if abs(x) as y > 5]
>
> as a simpler alternative to
>
>     [y for y in (abs(x) for x in numbers) if y > 5]
>
> but I'd be much more likely to write
>
>     results = []
>     for x in numbers:
>         y = abs(x)
>         if y > 5:
>             results.append(y)
>
> or
>
>     def bounded(it, lower):
>         for val in it:
>             absval = abs(val)
>             if absval > lower:
>                 yield absval
>
>     list(bounded(numbers, 5))
>
>
Again, fair. Not sure I can quite articulate why I would prefer a
comprehension here.
A very weak argument would be that such code tends to change, and when it
does it may get morphed back into something that *can* be done with a
comprehension, but you might end up leaving it as a for-loop with append.


> Obviously real world examples would be better than artificial ones, as
> artificially simple examples make terse notation look better... And in
> the example using a generator, you'd be able to give it a far more
> meaningful name with a bit of real-life domain terminology


Yeah agreed.
I would note that in a real-world example you are giving a name to
something that is forced to calculate *and* filter. So your name is going
to end-up being something like, say "is_registered_and_voting_for", or
"is_highest_tax_bracket_and_is_taxed". Which you might not actually write,
and instead opt for something like "tax_amount". Even in your code for my
artificial example your generator is named "bounded" but that does not
sound like it is doing any filtering at all, it sounds like it is simply
bounding all values. Of course to be fair, I didn't give a name for that at
all, and probably I want to give a name for the resulting list. Although of
course we both need to do that, but at least with a comprehension you only
have to come up with a name for the result, not for the result and the
associated generator.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20160308/fd8163fb/attachment.html>