permu.py

def alphabet_range(start_with, limit):
	alphabet = '$abcdefghijklmnopqrstuvwxyz' # alphabet set, don't modify it
	i = 0
	n = 0
	while n < limit: # from 1 to some number
		w = []
		k = i
		while k > 0:
			w.append(alphabet[k % 27]) # shortcut for enumeration, covert to 27-ary number
			k = k / 27
		i += 1
		if w.count('$') > 0:
			continue
		w.reverse() # reverse the list (otherwise will be aa, ba, ca, da, strange)
		word = ''.join(w) # generate string
		if start_with is not None and word != start_with: #check if it is start string, if not, continue
			continue
		start_with = None # if it is, start from here, output every string
		n += 1
		yield word # output generated string

for word in alphabet_range('abc', 1000):
	print word