If the authenticator object passed to the IMAP authenticate method tries to abort the handshake by returning None, TypeError is raised instead of sending the * line to the server.

>>> import imaplib
>>> imap = imaplib.IMAP4_SSL('imap.gmail.com')
>>> imap.authenticate(b'PLAIN', lambda x: None)
Traceback (most recent call last):
  File "<pyshell#5>", line 1, in <module>
    imap.authenticate(b'PLAIN', lambda x: None)
  File "C:\Python34\lib\imaplib.py", line 380, in authenticate
    typ, dat = self._simple_command('AUTHENTICATE', mech)
  File "C:\Python34\lib\imaplib.py", line 1133, in _simple_command
    return self._command_complete(name, self._command(name, *args))
  File "C:\Python34\lib\imaplib.py", line 940, in _command
    self.send(literal)
  File "C:\Python34\lib\imaplib.py", line 276, in send
    self.sock.sendall(data)
  File "C:\Python34\lib\ssl.py", line 723, in sendall
    v = self.send(data[count:])
  File "C:\Python34\lib\ssl.py", line 684, in send
    v = self._sslobj.write(data)
TypeError: 'str' does not support the buffer interface

The problem is that _Authenticator.process returns a string instead of bytes in this case.

Thanks for the report and the patch, Craig. Could you convert your reproducer to a test case? imaplib tests are located in Lib/test/test_imaplib.py.

New patch is attached.

The patch looks good to me, except that I think it would be better if the test verified that the '*' response is received by the server (perhaps return OK if we don't get it?)

Okay, I attached another patch.  The new version of the test returns success if the client's response is anything other than the abort line (*\r\n).

It fails with the current version of imaplib, fails if I change _Authenticator.process to output a byte-string besides *, and succeeds with the change in the patch.

Looks good to me, thanks.

New changeset fe55a36a335b by Robert Collins in branch '3.4':
Issue #23779: imaplib raises TypeError if authenticator tries to abort.
https://hg.python.org/cpython/rev/fe55a36a335b

New changeset b6f04b9d8c12 by Robert Collins in branch '3.5':
Issue #23779: imaplib raises TypeError if authenticator tries to abort.
https://hg.python.org/cpython/rev/b6f04b9d8c12

New changeset 0879f2c53289 by Robert Collins in branch 'default':
Issue #23779: imaplib raises TypeError if authenticator tries to abort.
https://hg.python.org/cpython/rev/0879f2c53289

Thanks for the patch. Applied to 3.4 through 3.6.