[Python-ideas] Return expressions

[Antony Lee]

This is fairly similar to a recent thread suggesting an "or raise"
construct ("x = f() or raise ValueError" as a shortcut for "x = f(); if not
x: raise ValueError").  I suggested that "raise" be made a value-less
expression, so that "x = f() or raise ..." can be intepreted normally.  As
a side effect, this would make lambdas slightly more powerful too.

Having "return" and "break" as value-less expressions would be nice too.
The interaction with nested loops in generators would be the natural one,
without any generator-specific syntax:

((x, y) if stay_in_inner(x, y) else break for x in X for y in Y)

is the same as

for x in X:
    for y in Y:
        if stay_in_inner(x, y):
            yield x, y
        else:
            break

or, with break-expression everywhere, as a separate generator:

for x in X:
    for y in Y:
        (yield x, y) if stay_in_inner(x, y) else break

whereas

((x, y) if stay_in_gen(x, y) else return for x in X for y in Y)

is the same as

for x in X:
    for y in Y:
        if stay_in_gen(x, y):
            yield x, y
        else:
            return

or

for x in X:
    for y in Y:
        (yield x, y) if stay_in_gen(x, y) else return

i.e. the intepretation of a genexp is still to take the nested "for"s and
"if"s after the leading expression, add suitable colons, newlines and
indentation, and insert the leading expression at the end.

Antony

2014-11-20 10:02 GMT-08:00 Nathaniel Smith <njs at pobox.com>:

> On Thu, Nov 20, 2014 at 4:36 PM, Chris Angelico <rosuav at gmail.com> wrote:
> > On Fri, Nov 21, 2014 at 3:21 AM, Nick Coghlan <ncoghlan at gmail.com>
> wrote:
> >> That does also suggest another possible alternative to the "or stop()"
> >> trick - allow *break* as an expression.
> >
> > Or just as a keyword component of a comprehension, in the same way
> > 'for' is. Since 'if' already has meaning, [x for x in range(10) if x
> >>= 5 break] would be confusing, so probably it'd have to be the
> > slightly-awkward-to-read [x for x in range(10) break x >= 5], adding a
> > termination condition to the preceding loop.
>
> 'break' is certainly better than 'return' -- currently 'break' means
> "look for the nearest enclosing loop", not "look for the enclosing
> function", so it'd preserve that at least.
>
> [x for x in range(10) if x >= 5 else break]?
>
> with 'else break' handled in the grammar as a non-compositional phrase
> like 'is not'? Not sure how this interacts with multiple mixed for and
> if clauses -- I've never wrapped my head around those semantics.
>
> -n
>
> --
> Nathaniel J. Smith
> Postdoctoral researcher - Informatics - University of Edinburgh
> http://vorpus.org
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