'Determine the filename containing the block info.
Return the file contents.'

def _get_block_contents(self, block_subdir_path, block_id):

    self._ensure_exported()
    block_file = self._get_block_filename(block_id)
    block_file_path = os.path.join(block_subdir_path, block_file)
    self.assertTrue(os.path.isfile(block_file_path), msg='{}    is    not    an    existing    file.'.format(block_file_path))
    with open(block_file_path, 'r') as file_handle:
        return file_handle.read()
