permutations.py

# Permutation function in python
# Computes all possible permutations of elements in a list

def permutations(lst):
    perms = []
    if len(lst) == 2:  #This is the stop condition
        perms.append(lst)
        permuted = lst[0:2] # We need to make a copy
        permuted[0], permuted[1] = permuted[1], permuted[0]
        perms.append(permuted)
        return perms
    for index, item in enumerate(lst):
        sublist = lst[:index] + lst[index+1 :]
        child_perms = permutations(sublist)
        for p in child_perms:
            p.insert(0, item)
            perms.append(p)
    return perms

# factorial 
def fact(i):
    if i == 0:
        return 1
    return i*fact(i-1)

test = ['a','b','c','d','e']
permus = permutations(test)
print permus
print fact(len(test)), len(permus)