def _download(self):
        """Download dataset by calling ``self.download()``
        if the dataset does not exists under ``self.raw_path``.

        By default ``self.raw_path = os.path.join(self.raw_dir, self.name)``
        One can overwrite ``raw_path()`` function to change the path.
        """
        if os.path.exists(self.raw_path):  # pragma: no cover
            return

        makedirs(self.raw_dir)
        self.download()
Download dataset by calling ``self.download()``
        if the dataset does not exists under ``self.raw_path``.

        By default ``self.raw_path = os.path.join(self.raw_dir, self.name)``
        One can overwrite ``raw_path()`` function to change the path.
        
python/dgl/data/dgl_dataset.py