'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):

    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
