[Python-ideas] Where-statement (Proposal for function expressions)

[MRAB]

Jan Kaliszewski wrote:
> 16-07-2009, 17:37 Daniel Stutzbach <daniel at stutzbachenterprises.com> wrote:
> 
>> On Thu, Jul 16, 2009 at 10:06 AM, Paul Moore <p.f.moore at gmail.com> wrote:
>>
>>> x[i] = 12 where:
>>>    i = some_complicated_expression()
>>>
>>> Was that deliberate? If so, could you explain why now, so that it's on
>>> record for the legions of people who will ask after it's implemented?
>>> :-)
>>
>> Because only the right-hand side is in the where-block's scope.
> 
> Yeah. But I believe it should be possible using global/nonlocal keywords,
> e.g.:
> 
> x[i] = 12 where:
>    global i
>    i = some_complicated_expression()
> 
> I think that many rules that apply for functions should also apply for
> where-blocks. (But obviously not all of them, e.g. return, yield and such
> stuff don't make sense)
> 
> 16-07-2009, 18:16 Chris Perkins <chrisperkins99 at gmail.com> wrote:
> 
>> Yes, I'm very interested to see what proportion of people find the
>> top-down style more readable.
> 
> I think even for the same person it can strongly depend on particular
> application. It'll be good for some things, and useless for other.
> But it isn't a strong argument against the feature. IMHO it'll be
> useful in many situations.
> 
> 15-07-2009, 14:49 Daniel Stutzbach <daniel at stutzbachenterprises.com> wrote:
> 
>> How about the following as additional syntactic sugar for the common
>> one-function case?
>> x = blah(f) where def f(item):
>>    body_of_f
> 
> +1 from me also.
> 
[Apologies for the word 'assignment' in what follows. :-)]

When defining a function, any name that's the target of an assignment
will default to being local; other names will be non-local.

In a 'where' construct the rule could be that any name that's the
target of an assignment before the 'where' will be non-local; other
names will default to local if they are the target of an assignment
within the 'where' construct, but non-local otherwise.


For example:

     bar = "outside"
     foo = bar where:
         bar = "inside"

'foo' is non-local. There's an assignment to 'bar' with the 'where' 
construct, so 'bar' is local in the statement and hides the non-local 'bar'.

Therefore 'foo' becomes bound to "inside".

More clearly:

     bar = "outside"
     local:
         nonlocal foo
         bar = "inside"
         foo = bar


Another example:

     x[i] = 12 where:
         i = some_complicated_expression()

'x' is non-local because there's no assignment to 'x' within the 'where'
construct'. 'i' is local because there is an assignment to 'i' within
the 'where' construct'.

More clearly:

     local:
         i = some_complicated_expression()
         x[i] = 12