while expression feature proposal

[Steven D'Aprano]

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.


-- 
Steven