Example:

>>> from poplib import POP3
>>> p = POP3("localhost")
>>> p.user("user")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/py3k/Lib/poplib.py", line 179, in user
    return self._shortcmd('USER %s' % user)
  File "/py3k/Lib/poplib.py", line 151, in _shortcmd
    self._putcmd(line)
  File "/py3k/Lib/poplib.py", line 98, in _putcmd
    self._putline(line)
  File "/py3k/Lib/poplib.py", line 91, in _putline
    self.sock.sendall('%s%s' % (line, CRLF))
TypeError: sendall() argument 1 must be string or buffer, not str
>>> p.user(b"user")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/py3k/Lib/poplib.py", line 179, in user
    return self._shortcmd('USER %s' % user)
  File "/py3k/Lib/poplib.py", line 151, in _shortcmd
    self._putcmd(line)
  File "/py3k/Lib/poplib.py", line 98, in _putcmd
    self._putline(line)
  File "/py3k/Lib/poplib.py", line 91, in _putline
    self.sock.sendall('%s%s' % (line, CRLF))
TypeError: sendall() argument 1 must be string or buffer, not str