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

[Phillip J. Eby]

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
>
>  >>> s = accum(0)
>  >>> s.next()

The initial next() will yield 0, not None.

>  >>> s.send(1)
>0

1

>  >>> s.send(5)
>1

6

>  >>> s.send(1)
>6

7


>Is the order of the output correct?  Is there a better way to write
>accum, that makes it feel more like the closure-based version?
>
>Is this for loop correct?
>
>  >>> s = accum(0)
>  >>> for i in s:
>...     if i >= 10: break
>...     print i,
>...     assert s.send(2) == i
>0 2 4 6 8

The assert will fail on the first pass.  s.send(2) will == i+2, e.g.:

 >>> s = accum(0)
 >>> for i in s:
...     if i>=10: break
...     print i,
...     assert s.send(2) == i+2
...
0 2 4 6 8