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

[Alex Light]

MRAB wrote:

> > Why use 'as'? Why not:

> i would use as because this whole where clause acts very similarly to a
context manager in that it sets a variable to a value for a small block

>
c = a + b with:

>     get_a(), get_b() as a, b

>
is equivilent to

> with get_a(), get_b() as a, b:

>     c = a + b

>
assuming get_a() and get_b() are context manager that return the values of a
and b and then delete them at the end

>
the thing with this though it that the get_a() and get_b() do not need to be
context managers the interpreter will create one so this will be valid:

>
c = a**2 with:

>     4 as a

>
i.e. it creates a context manager whose __enter__ method returns 4 and whose
exit method deletes a from the current namespace

>

strula moldan wrote:
>That will not work, the parser would think like this:
>
>c = sqrt(a*a + b*b) with:
>    get_a(), (get_b() as a), b

sorry haven't used 'as' in a while and i forgot that you
could not use it like you can everything else in compound
assignments (BTW why isn't a, b as c, d allowed it makes sense
especially since a, b = c, d is allowed it should be changed so it does)

>>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.

i think you miss the point which is understandable considering such a short
example
but consider if there are many many steps used to arrive at the variable.
you can give it a simple, descriptive name and nobody needs to see how you
got it unless they want to.
or a situation where you have many many variables in the with statement and
it is possible to understand from their names what they represent.

i.e.
sea = water()
with (get_temperature(sea) as temp, get_depth(sea) as depth, get_purity(sea)
as purity,
        get_salinity(sea) as saltiness, get_size(sea) as size #etc for a few
more lines
        get_density(sea) as density):
    sea_num = temp**depth + purity - salinity * size - density # one line
only

is much harder to understand than simply

sea_num = temp**depth + purity - salinity * size - density with: # one line
only
    get_temperature(sea) as temp, get_depth(sea) as depth, get_purity(sea)
as purity,
    get_salinity(sea) as saltiness, get_size(sea) as size #etc for a few
more lines
    get_density(sea) as density

the meaning of all the words is obvious (assuming you know that sea_num has
to do with the sea and water) and since it is you do not really need to have
all that clutter up above and can just put it below

>

On Tue, Jul 20, 2010 at 3:15 PM, MRAB <python at mrabarnett.plus.com> wrote:

>
 Alex Light wrote:
>
>> i would suggest overloading the 'with', 'as' combo
>>
>>>
>> c = sqrt(a*a + b*b) with:
>>
>>>    get_a(), get_b() as a, b
>>
>>>
>> or
>>
>>>
>>
>> c = sqrt(a*a + b*b) with:
>>
>>>    get_a() as a
>>
>>>    get_b() as b
>>
>> Why use 'as'? Why not:
>
>>
>
>    c = sqrt(a*a + b*b) with:
>
>>        a = get_a()
>
>>        b = get_b()
>
>>
> which is like:
>
>>
>    def _():
>
>>        a = get_a()
>
>>        b = get_b()
>
>>
>        return sqrt(a*a + b*b)
>
>>
>    c = _()
>
>>    del _
>
>>
>
>> it reads well and plus it follows since this statement acts similarly to a
>> regular with and as statement with the behavior of the context manager being
>> (in psudocode): set up: store original value of variable if any and set
>> variable to new value.
>>
>>> tear down: set value back to original or delete from local namespace if
>> it never had one
>>
>>>
>> additionally we do not need to introduce any new keywords
>>
>>>
>> any way that this is implemented though it would be quite useful
>>
>>>
>> On Tue, Jul 20, 2010 at 2:35 PM, George Sakkis <george.sakkis at gmail.com<mailto:
>> george.sakkis at gmail.com>> wrote:
>>
>>>
>>    On Tue, Jul 20, 2010 at 8:29 PM, Daniel Stutzbach
>>
>>>    <daniel at stutzbachenterprises.com
>>
>>>    <mailto:daniel at stutzbachenterprises.com>> wrote:
>>
>>>
>>     > On Tue, Jul 20, 2010 at 1:16 PM, Guido van Rossum
>>
>>>    <guido at python.org <mailto:guido at python.org>> wrote:
>>
>>>     >>
>>
>>>     >> Given the large number of Python users familiar with SQL compared
>> to
>>
>>>     >> those familiar with Haskell, I think we'd do wise to pick a
>>
>>>    different
>>
>>>     >> keyword instead of 'where'. I can't think of one right now though.
>>
>>>     >
>>
>>>     > Taking a cue from mathematics, how about "given"?
>>
>>>     >
>>
>>>     > c = sqrt(a*a + b*b) given:
>>
>>>     >     a = retrieve_a()
>>
>>>     >     b = retrieve_b()
>>
>>>
>>    Or if we'd rather overload an existing keyword than add a new one,
>>
>>>    "with" reads well too.
>>
>> _______________________________________________
>
>> Python-ideas mailing list
>
>> Python-ideas at python.org
>  http://mail.python.org/mailman/listinfo/python-ideas
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20100720/068fec3a/attachment.html>