Password generator

passwd-generator.py

# Write a password generator that meets these four criteria:
# 1- The password length is always 4
# 2- The password should consist letters and numbers from {A-F}, {a-f} and {1-6} and pick at least one from each of these(randomly)
# 3- No duplicate passwords generated
# 4- The password is generated totally randomly  

array = list('ABCDEFabcdef123456')
count = 0

def print_passwd(passwd):
    global count
    count += 1
    print passwd


def generate(chars):
    mapping = {}    
    for char in chars:
        mapping[char] = 0
    string = ''

    for key in mapping.keys():
        if mapping[key] == 0:
            string += key
            mapping[key] = 1
            if len(string) == len(mapping):
                print_passwd(string)

            gen(mapping, string)

            mapping[key] = 0
            string = string[:-1]

for i in xrange(0, 6):
    for j in xrange(6, 12):
        for k in xrange(12, 18):
            for l in xrange(0, 18):
                if l != i and l != j and l != k:
                    chars = [array[i], array[j], array[k], array[l] ]
                    generate(chars)             


print 'Total passwords generated is %s' % count

what does the below line do?
gen(mapping, string)