Also you will note that the return of the empty list for an empty line is exactly what you want for wrapping multiple line-break-delimited paragraphs.  Consider:

  >>> doc = "a para\nanother para\n\na third, but with an extra blank line between\n"
  >>> for line in doc.splitlines():
  ...    print('\n'.join(textwrap.wrap(line, width=5)))
  ...    if line:
  ...       print()
  a
  para

  anoth
  er
  para


  a thi
  rd,
  but
  with
  an
  extra
  blank
  line 
  betwe
  en

In other words, we need to add a blank line after our formatted paragraph, unless it is empty, in which case we don't want to add one or we'll have an extra.  This assumes that single-line-paragraphs do not have blank lines between them...if they do, then the algorithm is even simpler, as you don't need the if.