Let's consider there is an email box with password which contains Cyrillic symbols. When loging in

  imaplib.IMAP4_SSL(host, port).login(login, password)

password field is passed as "кириллица" (type is str, not unicode).
Then, I get this traceback:

  File "/usr/lib64/python2.7/imaplib.py", line 518, in login
    typ, dat = self._simple_command('LOGIN', user, self._quote(password))
  File "/usr/lib64/python2.7/imaplib.py", line 1083, in _simple_command
    return self._command_complete(name, self._command(name, *args))
  File "/usr/lib64/python2.7/imaplib.py", line 870, in _command
    self.send('%s%s' % (data, CRLF))
  File "/usr/lib64/python2.7/imaplib.py", line 1191, in send
    sent = self.sslobj.write(data)
  File "/usr/lib64/python2.7/ssl.py", line 669, in write
    return self._sslobj.write(data)

This is because self._sslobj.write method requires unicode string.

In other case, if password field is passed as u"кириллица" (type unicode, not str), I get another traceback:

  File "/usr/lib64/python2.7/imaplib.py", line 518, in login
    typ, dat = self._simple_command('LOGIN', user, self._quote(password))
  File "/usr/lib64/python2.7/imaplib.py", line 1083, in _simple_command
    return self._command_complete(name, self._command(name, *args))
  File "/usr/lib64/python2.7/imaplib.py", line 852, in _command
    data = '%s %s' % (data, self._checkquote(arg))

It's because '%s' % u"some unicode string" doesn't work.

I guess the problem was described in details.
The problem is that two different libraries are using different string types but they must be agreed.

You have cut off the exception message. Please provide a full traceback including the exception name and message.

Password algorithms typically handle only bytes. The encoding depends on your target system and is usually utf-8.

Thank you for fast reply.
Here's full traceback for first case:

Traceback (most recent call last):
  File "some_my_file.py", line 10, in some_function
    self.mail.login(login, password)
  File "/usr/lib64/python2.7/imaplib.py", line 518, in login
    typ, dat = self._simple_command('LOGIN', user, self._quote(password))
  File "/usr/lib64/python2.7/imaplib.py", line 1083, in _simple_command
    return self._command_complete(name, self._command(name, *args))
  File "/usr/lib64/python2.7/imaplib.py", line 870, in _command
    self.send('%s%s' % (data, CRLF))
  File "/usr/lib64/python2.7/imaplib.py", line 1191, in send
    sent = self.sslobj.write(data)
  File "/usr/lib64/python2.7/ssl.py", line 669, in write
    return self._sslobj.write(data)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 30-39: ordinal not in range(128)

Full traceback for second case:

Traceback (most recent call last):
  File "some_my_file.py", line 10, in some_function
    self.mail.login(login, password)
  File "/usr/lib64/python2.7/imaplib.py", line 518, in login
    typ, dat = self._simple_command('LOGIN', user, self._quote(password))
  File "/usr/lib64/python2.7/imaplib.py", line 1083, in _simple_command
    return self._command_complete(name, self._command(name, *args))
  File "/usr/lib64/python2.7/imaplib.py", line 852, in _command
    data = '%s %s' % (data, self._checkquote(arg))
UnicodeDecodeError: 'ascii' codec can't decode byte 0xd0 in position 1: ordinal not in range(128)


But okay, even if encoding depends on target system and it's usually utf-8 so why string operations in imaplib are not in usual format but in some other?

This is not a bug in Python's imaplib but a limitation of the LDAP protocol. It's not possible to enable UTF-8 mode before the session is authenticatd. LOGIN supports only ASCII user name and password. You have to use SASL PLAIN handshake with AUTHENTICATE, see https://tools.ietf.org/html/rfc6855.html#page-5

This is not a bug in Python's imaplib but a limitation of the LDAP protocol. It's not possible to enable UTF-8 mode before the session is authenticatd. LOGIN supports only ASCII user name and password. You have to use SASL PLAIN handshake with AUTHENTICATE, see https://tools.ietf.org/html/rfc6855.html#page-5

It works. Thanks a lot!