def get_password(self, username, acceptable_formats, properties):
<EOL>return None, None<EOL>
Get the password for user authentication.

        By default returns (None, None) providing no password. Should be
        overridden in derived classes unless only `check_password` functionality
        is available.

        :Parameters:
            - `username`: the username for which the password is requested.
            - `acceptable_formats`: a sequence of acceptable formats of the
              password data. Could be "plain" (plain text password),
              "md5:user:realm:password" (MD5 hex digest of user:realm:password)
              or any other mechanism-specific encoding. This allows
              non-plain-text storage of passwords. But only "plain" format will
              work with all password authentication mechanisms.
            - `properties`: mapping with authentication properties (those
              provided to the authenticator's ``start()`` method plus some
              already obtained via the mechanism).
        :Types:
            - `username`: `unicode`
            - `acceptable_formats`: sequence of `unicode`
            - `properties`: mapping

        :return: the password and its encoding (format).
        :returntype: `unicode`,`unicode` tuple.