Python imaplib kerberos mixin

kerberos.py

import base64
import kerberos
import imaplib


class IMAPGSSMixin(object):
    def gss_cb(self,challenge):
        input_token = base64.b64encode(challenge).decode('utf-8')

        if self.step == 0:
            ret = kerberos.authGSSClientStep(self._gss_context, input_token)
            if ret != kerberos.AUTH_GSS_CONTINUE:
                self.step = 1
        elif self.step == 1: 
            ret = kerberos.authGSSClientUnwrap(self._gss_context, input_token)
            response = kerberos.authGSSClientResponse(self._gss_context)
            ret = kerberos.authGSSClientWrap(self._gss_context, response, self.user)
        response = kerberos.authGSSClientResponse(self._gss_context)

        if response is None:
            return ""
        else:
            return base64.b64decode(response)

    def login_gssapi(self, user):
        self.user = user
        self.step = 0
        _unused, self._gss_context = kerberos.authGSSClientInit("imap@{}".format(self.host))
        return self.authenticate(b"GSSAPI", self.gss_cb)


class IMAP4(imaplib.IMAP4_SSL, IMAPGSSMixin):
    pass


# M.login_gssapi("USERNAME")