[Python-ideas] Yield-From: Finalization guarantees

[Guido van Rossum]

On Thu, Apr 2, 2009 at 5:35 PM, Ron Adam <rrr at ronadam.com> wrote:
>
>
> Jacob Holm wrote:
>
>> That might be the prevailing wisdom concerning GeneratorExit, at least
>> partly based on the fact that the only way to communicate anything useful
>> out of a closing generator is to raise another exception.   Thinking a bit
>> about coroutines, it would be nice to use "send" for the normal
>> communication and "close" to shut it down and getting a final result.
>>  Example:
>>
>> def averager():
>>   count = 0
>>   sum = 0
>>   while 1:
>>       try:            val = (yield)
>>       except GeneratorExit:
>>           return sum/count
>>       else:
>>           sum += val
>>           count += 1
>>
>> avg = averager()
>> avg.next() # start coroutine
>> avg.send(1.0)
>> avg.send(2.0)
>> print avg.close()  # prints 1.5
>>
>>
>> To do something similar today requires either a custom exception, or the
>> use of special values to tell the generator to yield the result.  I find
>> this version a lot cleaner.
>
> This doesn't seem less cleaner than the above to me.
>
> def averager():
>    sum = 0
>    count = 0
>    try:
>        while 1:
>            sum += yield
>            count += 1
>    finally:
>        yield sum / count
>
> avg = averager()
> avg.next()
> avg.send(1.0)
> avg.send(2.0)
> print avg.next()   # prints 1.5

But your version isn't clean -- it relies on "sum += yield" raising a
TypeError when yield returns None (due to .next() being the same as
.send(None)).

That's not to say I like Jacob's version that much, but I now
understand his use case. I note that Dave Beazley works around this
carefully in his tutorial (dabeaz.com/coroutines/) by using examples
that produce output on stdout -- and much later, in his multitasking
schedule example, his trampoline actually interprets yielding a value
that is neither a SystemCall instance nor a generator as a return from
a generator. (This is similar to the abuse that your example is giving
yield, actually.) I'll have to ponder this more.

__________
PS. Somehow the headers in your email made my reply add this:

  Python-Ideas <public-python-ideas-+ZN9ApsXKcEdnm+yROfE0A at ciao.gmane.org>,
Nick Coghlan <public-ncoghlan-Re5JQEeQqe8AvxtiuMwx3w at ciao.gmane.org>

Whoever did that, and whatever they did to cause it, please don't do it again.

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)