while expression feature proposal

[Terry Reedy]

On 10/25/2012 6:50 AM, Steven D'Aprano wrote:
> On Thu, 25 Oct 2012 11:52:31 +0200, Thomas Rachel wrote:
>
>> Am 25.10.2012 06:50 schrieb Terry Reedy:
>>
>>> Keep in mind that any new syntax has to be a substantial improvement in
>>> some sense or make something new possible. There was no new syntax in
>>> 3.2 and very little in 3.3.
>>
>> I would consinder this at least as new substantial than
>>
>>       yield_from it
>>
>> as opposed to
>>
>>       for i in it: yield i
>>
>> - although I think that was a good idea as well.
>
> Then I think you have misunderstood the purpose of "yield from". The fact
> that you can replace the two lines:
>
> for value in another_iterator:
>      yield iterator
>
> with a one-liner "yield from another_iterator" is the least important use-
> case for yield-from. If that was the only use-case, it probably would not
> have been allowed, because it adds complication to the language for a
> trivial gain.
>
> The purpose of yield-from is to transfer control to another coroutine,
> not to save one trivial line of code.
>
> [quote]
> However, if the subgenerator is to interact properly with the caller in
> the case of calls to send(), throw() and close(), things become
> considerably more difficult. As will be seen later, the necessary code is
> very complicated, and it is tricky to handle all the corner cases
> correctly.
>
> A new syntax will be proposed to address this issue. In the simplest use
> cases, it will be equivalent to the above for-loop, but it will also
> handle the full range of generator behaviour, and allow generator code to
> be refactored in a simple and straightforward way.
> [end quote]
>
> http://www.python.org/dev/peps/pep-0380/
>
>
> "yield from" is a *huge* win in terms of correctness and power, not just
> a trivial saving in lines of code. "while expr as var" is not.

r = yield from g

is equivalent to about 40 lines of code as given here
http://www.python.org/dev/peps/pep-0380/#formal-semantics

It took the developers several tries to first get a version that worked 
and then to work out the exact details.

-- 
Terry Jan Reedy