按字典顺序排列的k-排列

Question

I'm trying to generate k-permutations (variations) in lexicographical (alphabetical) order. For example, this code

import itertools

a = list('ABCD')
k = 2

for c in itertools.combinations(a, k):
    for p in itertools.permutations(c):
        print "".join(p),

prints

AB BA AC CA AD DA BC CB BD DB CD DC

and I'm looking for

AB AC AD BA BC BD CA CB CD DA DB DC

The answer needs to be iterable, so sort is not an option.

Answer 1

You can just use permutations without combinations :

import itertools

a = 'ABCD'
k = 2

for p in itertools.permutations(a, k):
    print "".join(p),

See also:

• Documentation for permutations(iterable[,r])

Answer 2

>>> ["".join(x) for x in itertools.permutations(a, k)]
['AB', 'AC', 'AD', 'BA', 'BC', 'BD', 'CA', 'CB', 'CD', 'DA', 'DB', 'DC']