To avoid confusing newbies, str() does not concatenate a list (or generator, etc.) of strings into a single string. You probably meant to call ''.join() instead.

  >>> import itertools
  >>> def f2_1(list):
  ...         return str(itertools.imap(chr, list))
  ... 
  >>> f2_1(range(10))
  '<itertools.imap object at 0x10048e850>'
  >>> def f2_2(list):
  ...         return str(chr(c) for c in list)
  ... 
  >>> f2_2(range(10))
  '<generator object <genexpr> at 0x10046b780>'