[Python-ideas] Fwd: Fwd: unpacking generalisations for list comprehension

[Steven D'Aprano]

On Fri, Oct 14, 2016 at 07:51:18AM +0000, Neil Girdhar wrote:
> Here's an interesting idea regarding yield **x:
> 
> Right now a function containing any yield returns a generator.  Therefore,
> it works like a generator expression, which is the lazy version of a list
> display.  lists can only contain elements x and unpackings *x.  Therefore,
> it would make sense to only have "yield x" and "yield *xs" (currently
> spelled "yield from xs")

No, there's no "therefore" about it. "yield from x" is not the same as 
"yield *x". 


*x is conceptually equivalent to replacing "*x" with a 
comma-separated sequence of individual items from x.

Given x = (1, 2, 3):


    f(*x) is like f(1, 2, 3)

    [100, 200, *x, 300] is like [100, 200, 1, 2, 3, 300]

    a, b, c, d = 100, *x is like a, b, c, d = 100, 1, 2, 3

Now replace "yield *x" with "yield 1, 2, 3". Conveniently, that syntax 
already works:

py> def gen():
...     yield 1, 2, 3
...
py> it = gen()
py> next(it)
(1, 2, 3)


"yield *x" should not be the same as "yield from x". Yielding a starred 
expression currently isn't allowed, but if it were allowed, it would be 
pointless: it would be the same as unpacking x, then repacking it into a 
tuple.

Either that, or we would have yet another special meaning for * 
unrelated to the existing meanings.




-- 
Steve