Python permutations with constraints

Question

I am using python 3 and I am trying to find a way to get all the permutations of a list while enforcing some constraints.

For instance, I have a list L=[1, 2,

Answer 1

def partial_permutations(*groups):
    groups = list(filter(None, groups)) # remove empties.
    # Since we iterate over 'groups' twice, we need to
    # make an explicit copy for 3.x for this approach to work.
    if not groups:
        yield []
        return
    for group in groups:
        for pp in partial_permutations(*(
             g[1:] if g == group else g
             for g in groups
        )):
            yield [group[0]] + pp