gistfile1.txt

>>> def check_portstr(portstr):
...  from_port, sep, to_port = portstr.partition('-')
...  if not to_port:
...   if not sep:
...    to_port = from_port
...   else:
...    raise Exception("onoes")
...  return from_port, to_port
... 
>>> check_portstr("1-2")
('1', '2')
>>> check_portstr("1")
('1', '1')
>>> check_portstr("1-")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 7, in check_portstr
Exception: onoes