Return a suitable command for verifying a file.

        :param signature_filename: The pathname to the file containing the
                                   signature.
        :param data_filename: The pathname to the file containing the
                              signed data.
        :param keystore: The path to a directory which contains the keys
                         used in verification. If not specified, the
                         instance's ``gpg_home`` attribute is used instead.
        :return: The verifying command as a list suitable to be
                 passed to :class:`subprocess.Popen`.

def get_verify_command(self, signature_filename, data_filename,
                           keystore=None):
        """
        Return a suitable command for verifying a file.

        :param signature_filename: The pathname to the file containing the
                                   signature.
        :param data_filename: The pathname to the file containing the
                              signed data.
        :param keystore: The path to a directory which contains the keys
                         used in verification. If not specified, the
                         instance's ``gpg_home`` attribute is used instead.
        :return: The verifying command as a list suitable to be
                 passed to :class:`subprocess.Popen`.
        """
        cmd = [self.gpg, '--status-fd', '2', '--no-tty']
        if keystore is None:
            keystore = self.gpg_home
        if keystore:
            cmd.extend(['--homedir', keystore])
        cmd.extend(['--verify', signature_filename, data_filename])
        logger.debug('invoking: %s', ' '.join(cmd))
        return cmd