seq_per.py

seq = ['A', 'B', 'C']

current = []
next = []
for char in seq:
    for c in current:
        next.append(c + char)
        next.append(char + c)
    next.append(char)
    current = current + next
    next = []


print(current)
# ['A', 'AB', 'BA', 'B', 'AC', 'CA', 'ABC', 'CAB', 'BAC', 'CBA', 'BC', 'CB', 'C']