[Python-ideas] 'where' statement in Python?

[Guido van Rossum]

On Tue, Jul 20, 2010 at 8:32 PM, Sturla Molden <sturla at molden.no> wrote:
> Den 20.07.2010 20:56, skrev Alex Light:
>>
>> i would suggest overloading the 'with', 'as' combo
>>
>> c = sqrt(a*a + b*b) with:
>>    get_a(), get_b() as a, b
>>
>
> That will not work, the parser would think like this:
>
> c = sqrt(a*a + b*b) with:
>    get_a(), (get_b() as a), b

Not true, we can define the grouping as we like. However if we do
something like this I'd rather use 'var = expr' instead of 'expr as
var'.

>> c = sqrt(a*a + b*b) with:
>>    get_a() as a
>>    get_b() as b
>>
>
> I think it tastes too much like functional programming. Specifically:
>
> It forces you to think backwards: it does not evaluate the lines in the
> order they read. The block below is evaluated before the expression above.
> It really means:
>
> a = get_a()
> b = get_b()
> c = sqrt(a*a + b*b) with:
> del a,b
>
> That I think is very annoying.

Like it or not, except for the keyword to use and the 'as' issue,
that's exactly the proposal (please read the PEP:
http://www.python.org/dev/peps/pep-3150/ ).

I personally like it; plus the "think backwards" idea is already used
in other parts of Python' syntax, e.g. list comprehensions. And of
course forward references work for method calls too.

> Why not just use context managers instead?
>
> with get_a() as a, get_b() as b:
>     c = sqrt(a*a + b*b)
>
> Now it reads the right way: top down, not bottom up.

Because this means something different and requires that get_a()
return something that obeys the context manager protocol.

-- 
--Guido van Rossum (python.org/~guido)