[Python-ideas] use "as" for block scope support

[海韵]

在 2011年7月26日 上午2:56，海韵 <lyricconch at gmail.com> 写道：
> 1.
> but what about ( t * s + t / s )? temp var always keep in memory.
> (should not reduce ( t + s ) to ( 2 * x ) since x, y may not a number.
> any operator here should be conside as  independent function. )
>
> 2.
> try convert this to the lambda form:
> (x+y as t, x-y as s)(t * s as u, t/s as v)(u << v as p, u>>v as q) p ** q
sorry, here should be :
try convert this to the lambda form:
R =  lamba x, y: (x+y as t, x-y as s)(t * s as u, t/s as v)(u << v as
p, u>>v as q) p ** q
using "as" can make lambda expression more graceful.
>
> In runtime:
> lambda = MAKE_FUNCTION/MAKE_CLOSURE =  PyFunction_New
> lambda call = CALL_FUNCTION =  PyFrame_New +  PyEval_EvalFrameEx
> all of them are heavy, (you can check the source code)
>
> but:
> as = PyCell_New
> as enter = SETUP_FINALLY = PyFrame_BlockSetup
> as leave = END_FINALLY = PyFrame_BlockPop
> which is much much much more more more cheaper than above.
>
> 2011/7/26 Bruce Leban <bruce at leapyear.org>:
>>
>>
>> On Mon, Jul 25, 2011 at 9:50 AM, 海韵 <lyricconch at gmail.com> wrote:
>>>
>>> Dupilcate K is used only for describe the scope rule.
>>> Advantage:
>>>
>>> 1.  the same grammer, the same behave
>>> "t" of  """(x+y as t, x-y as s) t * s + s + t""" is only valid in the
>>> expression which require
>>> just as
>>> "e" of """except ... as e""" only live in the suite which it belongs
>>>
>>> 2. readability includes laconic
>>> a = (x+y as t, x-y as s) t * s + s + t
>>> it doesnt break thinking. from left to right, people finish thinking
>>> inplace.
>>> temp var will not keep in memory, people can forget them since they
>>> are not valid outside the expression. everything is clean.
>>> t, s = x+y,  x-y
>>> a = t * s + s + t
>>> in this case, thinking is break and temp var keep in memory,
>>> people should take some care about it (for example, in nested loop)
>>
>> (x + y) * (x - y) + 2 * x
>> is IMHO more readable - especially note the absence of y in the third term.
>>>
>>> 3. this enchant the "lambda" syntax
>>> list.sort(key = lambda p: (sqrt(p.x**2+p.y**2) as r) r**2+ A*r + B)
>>
>> I don't know what "enchant" means here, but you can do this if you want:
>> list.sort(key = lambda p: (lambda r: r**2 + A*r + B) (sqrt(p.x**2+p.y**2)))
>> I'm not recommending that as I think it would be more readable to name that
>> function so that it would be more clear why you're using that function as
>> the sort key. But, to me, this example does not justify new syntax.
>> --- Bruce
>