permurations.py

import itertools
list(itertools.permutations('ABCD', 3))

[('A', 'B', 'C'),
 ('A', 'B', 'D'),
 ('A', 'C', 'B'),
 ('A', 'C', 'D'),
 ('A', 'D', 'B'),
 ('A', 'D', 'C'),
 ('B', 'A', 'C'),
 ('B', 'A', 'D'),
 ('B', 'C', 'A'),
 ('B', 'C', 'D'),
 ('B', 'D', 'A'),
 ('B', 'D', 'C'),
 ('C', 'A', 'B'),
 ('C', 'A', 'D'),
 ('C', 'B', 'A'),
 ('C', 'B', 'D'),
 ('C', 'D', 'A'),
 ('C', 'D', 'B'),
 ('D', 'A', 'B'),
 ('D', 'A', 'C'),
 ('D', 'B', 'A'),
 ('D', 'B', 'C'),
 ('D', 'C', 'A'),
 ('D', 'C', 'B')]

seq = 'ABCD'
perm = ((x, y, z) for x in seq
                  for y in seq if y != x 
                  for z in seq if z != x and z != y)
next(perm)
next(perm)
next(perm)