>>> def construct(word, alphabet):     
...     if len(word)<5:
...             for x in alphabet:
...                     print word+x  
...                     construct(word+x,alphabet)
...     
... 
>>> alphabet=['a','b','c']
>>> word=''
>>> construct(word, alphabet)
a
aa
aaa
aaaa
aaaaa
aaaab
...
ccccc