[Python-Dev] PEP 342: simple example, closure alternative

Thu Aug 25 22:12:35 CEST 2005

Phillip J. Eby wrote:
> At 02:10 PM 8/25/2005 -0500, Ian Bicking wrote:
>>>I was trying to translate a pattern that uses closures in a language
>>like Scheme (where closed values can be written to) to generators using
>>PEP 342, but I'm not clear exactly how it works; the examples in the PEP
>>have different motivations. Since I can't actually run these examples,
>>perhaps someone could confirm or debug these:
>>>>A closure based accumulator (using Scheme):
>>>>(define (accum n)
>> (lambda (incr)
>> (set! n (+ n incr))
>> n))
>>(define s (accum 0))
>>(s 1) ; -> 1 == 0+1
>>(s 5) ; -> 6 == 1+5
>>>>So I thought the generator version might look like:
>>>>def accum(n):
>> while 1:
>> incr = (yield n) or 0
>> n += incr

Bah, I don't know why this had me so confused. Well, I kind of know 
why. So maybe this example would be better written:
def accum(n):
 incr = yield # wait to get the first incr to be sent in
 while 1:
 n += incr
 incr = yield n # return the new value, wait for next incr
This way it is more explicit all around -- the first call to .next() is 
just setup, kind of like __init__ in an object, except it has to be 
explicitly invoked.
-- 
Ian Bicking / ianb at colorstudy.com / http://blog.ianbicking.org