Why "one line"? You can fit anything onto one line.
Assuming you want them to start with 'a', and increment by one character each time (with wrapping > 26), here's a line:
>>> mkstring = lambda(x): "".join(map(chr, (ord('a')+(y%26) for y in range(x))))
>>> mkstring(10)
'abcdefghij'
>>> mkstring(30)
'abcdefghijklmnopqrstuvwxyzabcd'