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genexp reduce syntax in 2.5a1


On the python3000 mailing list there was some discussion of a "comprehension
syntax" for reduce.

This inspired me the following proof-of-concept in pure python 2.5a1, although I
don't know if it relies on an intended feature or a(n unintended) bug.

Cheers, BB
--

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 :
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
Apr 18 '06 #1
0 937

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