def get_credential(self, service, username):
<EOL>if username is not None:<EOL><INDENT>password = self.get_password(service, username)<EOL>if password is not None:<EOL><INDENT>return credentials.SimpleCredential(<EOL>username,<EOL>password,<EOL>)<EOL><DEDENT><DEDENT>return None<EOL>
Gets the username and password for the service.
        Returns a Credential instance.

        The *username* argument is optional and may be omitted by
        the caller or ignored by the backend. Callers must use the
        returned username.