给定列表中项目的组合

>>> l = ['yahoo', 'wikipedia', 'freebase']
>>> import itertools
>>> for i in range(1, len(l) +1):
    print(list(itertools.combinations(l, r=i)))


[('yahoo',), ('wikipedia',), ('freebase',)]
[('yahoo', 'wikipedia'), ('yahoo', 'freebase'), ('wikipedia', 'freebase')]
[('yahoo', 'wikipedia', 'freebase')]

P.S.为什么是这个wiki?

它被称为动力集。这是来自 itertools docs :

def powerset(iterable):
    "powerset([1,2,3]) --> () (1,) (2,) (3,) (1,2) (1,3) (2,3) (1,2,3)"
    s = list(iterable)
    return chain.from_iterable(combinations(s, r) for r in range(len(s)+1))

你基本上是从1数到2 ⁿ - 1 二进制 :

0 0 1    ['freebase']
0 1 0    ['wikipedia']
0 1 1    ['wikipedia', 'freebase']
1 0 0    ['yahoo']
1 0 1    ['yahoo', 'freebase']
1 1 0    ['yahoo', 'wikipedia']
1 1 1    ['yahoo', 'wikipedia', 'freebase']

这就是所谓的动力装置。跟着这个走 algorithm . 下面是一个简单的实现:

def powerset(seq):
  if len(seq):
    head = powerset(seq[:-1])
    return head + [item + [seq[-1]] for item in head]
  else:
    return [[]]

>>> powerset(['yahoo', 'wikipedia', 'freebase'])
[[], ['yahoo'], ['wikipedia'], ['yahoo', 'wikipedia'], ['freebase'], ['yahoo', 'freebase'], ['wikipedia', 'freebase'], ['yahoo', 'wikipedia', 'freebase']]

还有一个:

def powerset(s):
  sets = []
  indicator = lambda x: x & 1
  for element in xrange(2**len(s)):
    n = element
    subset = []
    for x in s:
        if indicator(n):
            subset.append(x)
        n >>= 1
    sets.append(subset)
  return sets