从多个具有重复列表的列表中生成所有组合

Question

I have multiple lists, some of them are repeats and I need all combinations, excluding ones where the same element from a repeated list is chosen. For example, I have

import itertools
list1 = [1,2,3]
list2 = [4,5,6]
list3 = [4,5,6]
list4 = [7,8,9]
a = [list1,list2,list3,list4]
print list(itertools.product(*a))

which outputs

(1,4,4,7)
(1,4,4,8)
(1,4,4,9)
(1,4,5,7)
.
.
.

etc, as you'd expect, but what I want it to do is output every combination, without repeating elements from lists 2 and 3. Like this:

(1,4,5,7)
(1,4,5,8)
(1,4,5,9)
(1,4,6,7)
(1,4,6,8)
(1,4,6,9)
(1,5,6,7)
(1,5,6,8)
(1,5,6,9)
(2,4,5,7)
.
.
.

I'd obviously like to avoid having to remove them manually after creating the list, but any help on how to do this efficiently is really appreciated. Thanks.

Answer 1

The easy way is a generator expression with a filter:

print list(item for item in itertools.product(*a) if item[1] != item[2])

If two items are considered the same if they contain the same elements, and if each item is guaranteed to contain no repeated elements, you can discard duplicates by changing them into set s and only adding them to your list if they're not already in it:

result = []
for item in itertools.product(*a):
    if item[1]==item[2]:
        continue
    item = set(item)
    if item not in result:
        result.append(item)

print result