how to convert a function into generator?

[Schüle Daniel]

Hello,

I came up with this algorithm to generate all permutations
it's not the best one, but it's easy enough

# lst = list with objects
def permute3(lst):
 tmp = []
 lenlst = len(lst)
 def permute(perm, level):
 if level == 1:
 tmp.append(perm)
 return
 for i in lst:
 if i not in perm:
 permute(perm + (i,), level - 1)
 for item in lst:
 permute((item,), lenlst)
 return tuple(tmp)

now I want to make a generator from it
the idea is to get each time a new permutation
I don't really understand how to handle the case
when my function has an inner function which has
a yield statement ..
I would say that the inner function becomes an generator
and stopsyields the value on each yield .. but in reality
I want to propogate these values to the caller of the outer function
I hope you got the idea of what I mean
the code is the sketch of the idea

def permute3gen(lst):
 lenlst = len(lst)
 def permute(perm, level):
 if level == 1:
 yield perm
 return # not sure return without a value is allowed, 
theoretically it could be replaces with if/else block
 for i in lst:
 if i not in perm:
 permute(perm + (i,), level - 1)
 for item in lst:
 yield permute((item,), lenlst) # makes generator from the outer 
function too


this is what I get

In [67]: reload permute
--- reload(permute)
Out[67]: module 'permute' from 'permute.pyc'

In [68]: p =  permute.permute3gen([a,b,c,d])

In [69]: p
Out[69]: generator object at 0x2af3a44795f0

In [70]: x = p.next()

In [71]: y = p.next()

In [72]: x
Out[72]: generator object at 0x2af3a448e830

In [73]: y
Out[73]: generator object at 0x2af3a448e878

In [74]: x.next()
---
type 'exceptions.StopIteration' Traceback (most recent call last)

/pool/PROG/python/permute/ipython console in module()

type 'exceptions.StopIteration':

I don't understand why the generator x raises StopIteration exception
I would expect that x.next() would call
permute((a,), 4) and would stop at abcd

thanks in advance

regards, Daniel
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Re: how to convert a function into generator?

[Carsten Haese]

On Wed, 2006-12-06 at 17:33 +0100, Schüle Daniel wrote:
 def permute3gen(lst):
  lenlst = len(lst)
  def permute(perm, level):
  if level == 1:
  yield perm
  return   # not sure return without a value is allowed, 
 theoretically it could be replaces with if/else block
  for i in lst:
  if i not in perm:
  permute(perm + (i,), level - 1)  ##1##
  for item in lst:
  yield permute((item,), lenlst) # makes generator from the outer 
 function too ##2##

Your only problem is in knowing what to do with recursively invoked
sub-generators. You have two such invocations, which I marked above with
##1## and ##2##, and neither one is handled correctly.

In ##1##, you construct a sub-generator and simply discard it. In ##2##,
you construct a sub-generator, and yield it (instead of yielding its
elements). In neither case do you actually consume any of the elements
that the sub-generators are prepared to produce.

To usefully invoke a sub-generator, you need to consume it (i.e. iterate
over it) and yield whatever it produces.

Hope this helps,

Carsten


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