4 examples of 'itertools combinations' in Python

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def combinations(c, r):
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    # combinations('ABCD', 2) --> AB AC AD BC BD CD
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    # combinations(range(4), 3) --> 012 013 023 123
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    pool = tuple(range(c))
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    n = len(pool)
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    if r > n:
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        return
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    indices = list(range(r))
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    yield tuple(pool[i] for i in indices)
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    while True:
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        for i in reversed(list(range(r))):
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            if indices[i] != i + n - r:
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                break
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        else:
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            return
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        indices[i] += 1
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        for j in range(i+1, r):
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            indices[j] = indices[j-1] + 1
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        yield tuple(pool[i] for i in indices)

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def testCombinations(self):
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    for i in range(15):
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        case = range(i)
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        k = random.randrange(len(case) + 1)
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        result = combinations(case, k)
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        result = set(map(tuple, result))
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        expect = itertools.combinations(case, k)
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        expect = set(expect)
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        self.assertEqual(result, expect)

51
def combinations(a, b):
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    """ Returns all combinations of
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     - fast.forwards->fast.backwards
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     - fast.forwards->slow.backwards
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     - slow.forwards->fast.backwards
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     - slow.forwards->slow.backwards
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    """
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    return list(itertools.chain.from_iterable(
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        [
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            list(itertools.product(*zip(*item)))
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            for item in correspondences(a, b)
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        ]
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    ))

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def all_combinations(items):
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    variants = []
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    for n in range(len(items)+1):
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        variants.extend(itertools.combinations(items, n))
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    return variants