[Python-3000] genexp syntax for reduce (already in 2.5a1)

[Guido van Rossum]

I wouldn't recommend this approach to anyone.

On 4/18/06, Boris Borcic <boris at batiment71.net> wrote:
> Hello,
>
> Since the notion of a "generator comprehension" reduce syntax for py3k was
> discussed on this list - it may have escaped attention that py2.5a1 already
> allows the following, admittedly not very efficient (nor too robust in this
> proof-of-concept implementation).
>
> hth, Boris Borcic
>
>
> def ireduce(gen) :
>      """
>      Generator expression syntax for reduce and generalizations
>
>      Usage examples:
>
>      - (yield) nothing as below if there is no seed value
>      *and* the expression consists of a single arithmetic op
>
>      >>> ireduce(x+(yield) for x in range(101)) # famous Gauss example
>      5050
>      >>> ireduce(x*(yield) for x in range(1,6))
>      120
>
>      - if there is a seed, yield it (unless it's None)
>
>      >>> ireduce(x*(yield 2) for x in range(1,6)) # 2*1*2*3*4*5
>      240
>      >>> ireduce(x*(yield 2) for x in range(2,6)) # 2*2*3*4*5
>      240
>      >>> ireduce(x*(yield 2) for x in range(6))   # 2*0*1*2*3*4*5
>      0
>
>      - if the seed is None, (yield INone) instead
>
>      >>> ireduce((x,(yield INone)) for x in "for all good men".split())
>      ('men', ('good', ('all', ('for', None))))
>
>      >>> ireduce([(yield INone),x] for x in "for all good men".split())
>      [[[[None, 'for'], 'all'], 'good'], 'men']
>
>      - do as below if you want no special seed and the operation to reduce
>      isn't a simple arithmetic operation
>
>      >>> ireduce((x,(yield Seed(x))) for x in "for all good men".split())
>      ('men', ('good', ('all', 'for')))
>
>      >>> ireduce(x+' '+(yield Seed(x)) for x in "for all good men".split())
>      'men good all for'
>
>      >>> ireduce({ x : (yield Seed(x))} for x in "for all good men".split())
>      {'men': {'good': {'all': 'for'}}}
>
>      - and notice these for a comparison
>
>      >>> ireduce({ x : (yield x)} for x in "for all good men".split())
>      {'men': {'good': {'all': {'for': 'for'}}}}
>
>      >>> ireduce({ x : (yield Seed(None))} for x in "for all good men".split())
>      {'men': {'good': {'all': None}}}
>      """
>
>      cumulate = gen.next()
>      if cumulate is None :
>          cumulate = Neutral()
>      elif cumulate is INone :
>          cumulate = None
>      elif isinstance(cumulate,Seed) :
>          cumulate = cumulate.seed
>          gen.send(cumulate)
>          gen.next()
>      try :
>          while True :
>              cumulate = gen.send(cumulate)
>              gen.next()
>      except StopIteration :
>          return cumulate
>
> class Neutral : # this would need reworking
>      def __coerce__(self,other) :
>          self.other = other
>          return (self,self)
>
>      def __getattr__(self,attr) :
>          return lambda *x,**y : self.__dict__['other']
>
> class INone : pass
>
> class Seed :
>      def __init__(self,seed) :
>          self.seed = seed
>
>
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--
--Guido van Rossum (home page: http://www.python.org/~guido/)