[Python-ideas] return value of yield expressions

[Guido van Rossum]

On Tue, Sep 13, 2011 at 10:57 AM, Georg Brandl <g.brandl at gmx.net> wrote:
> Am 13.09.2011 19:34, schrieb Guido van Rossum:
>> On Tue, Sep 13, 2011 at 10:21 AM, H. Krishnan <hetchkay at gmail.com> wrote:
>>>> > (a1, a2, a3 = 3, *args, **kwds) = (yield <expr>)
>>>>
>>>> What on earth is this syntax supposed to mean? Never mind that there's
>>>> a yield on the RHS; it is just a parenthesized expression so it could
>>>> could be any other function. What on earth do you expect to happen
>>>> with the syntax on the left, i.e. with the part
>>>
>>>
>>>>
>>>> (a1, a2, a3 = 3, *args, **kwds) = ........whatever........
>>>
>>> As I said "forgeting the backward compatibility issue".
>>
>> It's not a backwards compatibility issue. Maybe I'm dense, but *I do
>> not understand what it means in your proposal.*
>>
>>> Suppose we use
>>> Jacob's syntax (for argument's sake)
>>
>>> *(a1, a2, a3 = 3, *args, **kwds) = (yield expr)
>>
>>> and with g.send(*a, **k) being called, a1, a2, a3, args, kwds can be
>>> inferred using the same semantics that is used to decipher a1, a2, a3, args,
>>> kwds in a call to the following function with *a and **k as arguments:
>>> def func(a1, a2, a3 = 3, *args, **kwds):
>>>    ...
>>
>> I still don't follow. Can you show a specific argument list to send,
>> e.g. g.send(1, 2, 3, foo='a', bar='b') and then tell me what the
>> values of a1, a2, a3, args and kwds will be? And, since
>>
>> X = Y
>>
>> should always be equivalent to
>>
>> tmp = Y
>> X = tmp
>>
>> can you also tell me what value is supposed to be produced by (yield
>> expr)? I.e. if I did this:
>>
>> tmp = (yield expr)
>> *(a1, a2, a3 = 3, *args, **kwds) = tmp
>>
>> what would the value of tmp be?
>>
>>> That (yield expr) can be used in expressions does not (I feel) affect this
>>> (particularly if we go with Jacob's suggestion of this being a general
>>> unpacking option), just in the same way that "a, b, *args, c = <tuple>" does
>>> not affect using tuples in expressions. But after reading your comment in
>>> another thread about those who don't know how python works should keep
>>> quiet, I guess it is best if I end this here :-)
>>
>> I understand "a, b, *c = <something>". I do not understand three
>> things in your example:
>>
>> *(....) = <something>  # What is the meaning of the prefix *?
>>
>> *(.... = .....) = <something>  # What does an = inside parentheses mean?
>>
>> *(.... = ......, **kwds) = <something>  # What does **kwds mean in this context?
>>
>
> Apparently that syntax would be exclusive for "= yield", which then "unpacks"
> the arguments given to generator.send() as if the LHS was the argument list
> of a function.

That sounds like impossible to implement. If there should be
special-case treatment it should use some other form of syntax (e.g.
"yield <expr> as <var>"). But I really don't see why this is so
important to change so much of the syntax of the language.

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