.. _guide-data-pipeline-download:

4.2 Download raw data (optional)
--------------------------------

If a dataset is already in local disk, make sure it’s in directory
``raw_dir``. If one wants to run the code anywhere without bothering to
download and move data to the right directory, one can do it
automatically by implementing function ``download()``.

If the dataset is a zip file, make ``MyDataset`` inherit from
:class:`dgl.data.DGLBuiltinDataset` class, which handles the zip file extraction for us. Otherwise,
one needs to implement ``download()`` like in :class:`~dgl.data.QM7bDataset`:

.. code:: 

    import os
    from dgl.data.utils import download
    
    def download(self):
        # path to store the file
        file_path = os.path.join(self.raw_dir, self.name + '.mat')
        # download file
        download(self.url, path=file_path)

The above code downloads a .mat file to directory ``self.raw_dir``. If
the file is a .gz, .tar, .tar.gz or .tgz file, use :func:`~dgl.data.utils.extract_archive`
function to extract. The following code shows how to download a .gz file
in :class:`~dgl.data.BitcoinOTCDataset`:

.. code:: 

    from dgl.data.utils import download, check_sha1
    
    def download(self):
        # path to store the file
        # make sure to use the same suffix as the original file name's
        gz_file_path = os.path.join(self.raw_dir, self.name + '.csv.gz')
        # download file
        download(self.url, path=gz_file_path)
        # check SHA-1
        if not check_sha1(gz_file_path, self._sha1_str):
            raise UserWarning('File {} is downloaded but the content hash does not match.'
                              'The repo may be outdated or download may be incomplete. '
                              'Otherwise you can create an issue for it.'.format(self.name + '.csv.gz'))
        # extract file to directory `self.name` under `self.raw_dir`
        self._extract_gz(gz_file_path, self.raw_path)

The above code will extract the file into directory ``self.name`` under
``self.raw_dir``. If the class inherits from :class:`dgl.data.DGLBuiltinDataset`
to handle zip file, it will extract the file into directory ``self.name`` 
as well.

Optionally, one can check SHA-1 string of the downloaded file as the
example above does, in case the author changed the file in the remote
server some day.
