.. _guide-data-pipeline-process:

4.3 Process data
----------------

:ref:`(中文版) <guide_cn-data-pipeline-process>`

One can implement the data processing code in function ``process()``, and it
assumes that the raw data is located in ``self.raw_dir`` already. There
are typically three types of tasks in machine learning on graphs: graph
classification, node classification, and link prediction. This section will show
how to process datasets related to these tasks.

The section focuses on the standard way to process graphs, features and masks.
It will use builtin datasets as examples and skip the implementations
for building graphs from files, but add links to the detailed
implementations. Please refer to :ref:`guide-graph-external` to see a
complete guide on how to build graphs from external sources.

Processing Graph Classification datasets
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Graph classification datasets are almost the same as most datasets in
typical machine learning tasks, where mini-batch training is used. So one can
process the raw data to a list of :class:`dgl.DGLGraph` objects and a list of
label tensors. In addition, if the raw data has been split into
several files, one can add a parameter ``split`` to load specific part of
the data.

Take :class:`~dgl.data.QM7bDataset` as example:

.. code:: 

    from dgl.data import DGLDataset

    class QM7bDataset(DGLDataset):
        _url = 'http://deepchem.io.s3-website-us-west-1.amazonaws.com/' \
               'datasets/qm7b.mat'
        _sha1_str = '4102c744bb9d6fd7b40ac67a300e49cd87e28392'
    
        def __init__(self, raw_dir=None, force_reload=False, verbose=False):
            super(QM7bDataset, self).__init__(name='qm7b',
                                              url=self._url,
                                              raw_dir=raw_dir,
                                              force_reload=force_reload,
                                              verbose=verbose)
    
        def process(self):
            mat_path = self.raw_path + '.mat'
            # process data to a list of graphs and a list of labels
            self.graphs, self.label = self._load_graph(mat_path)
        
        def __getitem__(self, idx):
            """ Get graph and label by index
    
            Parameters
            ----------
            idx : int
                Item index
    
            Returns
            -------
            (dgl.DGLGraph, Tensor)
            """
            return self.graphs[idx], self.label[idx]
    
        def __len__(self):
            """Number of graphs in the dataset"""
            return len(self.graphs)


In ``process()``, the raw data is processed to a list of graphs and a
list of labels. One must implement ``__getitem__(idx)`` and ``__len__()``
for iteration. DGL recommends making ``__getitem__(idx)`` return a
tuple ``(graph, label)`` as above. Please check the `QM7bDataset source
code <https://docs.dgl.ai/en/0.5.x/_modules/dgl/data/qm7b.html#QM7bDataset>`__
for details of ``self._load_graph()`` and ``__getitem__``.

One can also add properties to the class to indicate some useful
information of the dataset. In :class:`~dgl.data.QM7bDataset`, one can add a property
``num_labels`` to indicate the total number of prediction tasks in this
multi-task dataset:

.. code:: 

    @property
    def num_labels(self):
        """Number of labels for each graph, i.e. number of prediction tasks."""
        return 14

After all these coding, one can finally use :class:`~dgl.data.QM7bDataset` as
follows:

.. code:: 

    import dgl
    import torch

    from dgl.dataloading import GraphDataLoader
    
    # load data
    dataset = QM7bDataset()
    num_labels = dataset.num_labels
    
    # create dataloaders
    dataloader = GraphDataLoader(dataset, batch_size=1, shuffle=True)
    
    # training
    for epoch in range(100):
        for g, labels in dataloader:
            # your training code here
            pass

A complete guide for training graph classification models can be found
in :ref:`guide-training-graph-classification`.

For more examples of graph classification datasets, please refer to DGL's builtin graph classification
datasets: 

* :ref:`gindataset`

* :ref:`minigcdataset`

* :ref:`qm7bdata`

* :ref:`tudata`

Processing Node Classification datasets
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Different from graph classification, node classification is typically on
a single graph. As such, splits of the dataset are on the nodes of the
graph. DGL recommends using node masks to specify the splits. The section uses
builtin dataset `CitationGraphDataset <https://docs.dgl.ai/en/0.5.x/_modules/dgl/data/citation_graph.html#CitationGraphDataset>`__ as an example:

In addition, DGL recommends re-arrange the nodes and edges so that nodes
near to each other have IDs in a close range. The procedure could improve
the locality to access a node's neighbors, which may benefit follow-up
computation and analysis conducted on the graph. DGL provides an API called
:func:`dgl.reorder_graph` for this purpose. Please refer to ``process()``
part in below example for more details.

.. code:: 

    from dgl.data import DGLBuiltinDataset
    from dgl.data.utils import _get_dgl_url
    
    class CitationGraphDataset(DGLBuiltinDataset):
        _urls = {
            'cora_v2' : 'dataset/cora_v2.zip',
            'citeseer' : 'dataset/citeseer.zip',
            'pubmed' : 'dataset/pubmed.zip',
        }
    
        def __init__(self, name, raw_dir=None, force_reload=False, verbose=True):
            assert name.lower() in ['cora', 'citeseer', 'pubmed']
            if name.lower() == 'cora':
                name = 'cora_v2'
            url = _get_dgl_url(self._urls[name])
            super(CitationGraphDataset, self).__init__(name,
                                                       url=url,
                                                       raw_dir=raw_dir,
                                                       force_reload=force_reload,
                                                       verbose=verbose)
    
        def process(self):
            # Skip some processing code
            # === data processing skipped ===
    
            # build graph
            g = dgl.graph(graph)
            # splitting masks
            g.ndata['train_mask'] = train_mask
            g.ndata['val_mask'] = val_mask
            g.ndata['test_mask'] = test_mask
            # node labels
            g.ndata['label'] = torch.tensor(labels)
            # node features
            g.ndata['feat'] = torch.tensor(_preprocess_features(features),
                                           dtype=F.data_type_dict['float32'])
            self._num_labels = onehot_labels.shape[1]
            self._labels = labels
            # reorder graph to obtain better locality.
            self._g = dgl.reorder_graph(g)
    
        def __getitem__(self, idx):
            assert idx == 0, "This dataset has only one graph"
            return self._g
    
        def __len__(self):
            return 1

For brevity, this section skips some code in ``process()`` to highlight the key
part for processing node classification dataset: splitting masks. Node
features and node labels are stored in ``g.ndata``. For detailed
implementation, please refer to `CitationGraphDataset source
code <https://docs.dgl.ai/en/0.5.x/_modules/dgl/data/citation_graph.html#CitationGraphDataset>`__.

Note that the implementations of ``__getitem__(idx)`` and
``__len__()`` are changed as well, since there is often only one graph
for node classification tasks. The masks are ``bool tensors`` in PyTorch
and TensorFlow, and ``float tensors`` in MXNet.

The section uses a subclass of ``CitationGraphDataset``, :class:`dgl.data.CiteseerGraphDataset`,
to show the usage of it:

.. code:: 

    # load data
    dataset = CiteseerGraphDataset(raw_dir='')
    graph = dataset[0]
    
    # get split masks
    train_mask = graph.ndata['train_mask']
    val_mask = graph.ndata['val_mask']
    test_mask = graph.ndata['test_mask']
    
    # get node features
    feats = graph.ndata['feat']
    
    # get labels
    labels = graph.ndata['label']

A complete guide for training node classification models can be found in
:ref:`guide-training-node-classification`.

For more examples of node classification datasets, please refer to DGL's
builtin datasets:

* :ref:`citationdata`

* :ref:`corafulldata`

* :ref:`amazoncobuydata`

* :ref:`coauthordata`

* :ref:`karateclubdata`

* :ref:`ppidata`

* :ref:`redditdata`

* :ref:`sbmdata`

* :ref:`sstdata`

* :ref:`rdfdata`

Processing dataset for Link Prediction datasets
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The processing of link prediction datasets is similar to that for node
classification’s, there is often one graph in the dataset.

The section uses builtin dataset
`KnowledgeGraphDataset <https://docs.dgl.ai/en/0.5.x/_modules/dgl/data/knowledge_graph.html#KnowledgeGraphDataset>`__
as an example, and still skips the detailed data processing code to
highlight the key part for processing link prediction datasets:

.. code:: 

    # Example for creating Link Prediction datasets
    class KnowledgeGraphDataset(DGLBuiltinDataset):
        def __init__(self, name, reverse=True, raw_dir=None, force_reload=False, verbose=True):
            self._name = name
            self.reverse = reverse
            url = _get_dgl_url('dataset/') + '{}.tgz'.format(name)
            super(KnowledgeGraphDataset, self).__init__(name,
                                                        url=url,
                                                        raw_dir=raw_dir,
                                                        force_reload=force_reload,
                                                        verbose=verbose)
    
        def process(self):
            # Skip some processing code
            # === data processing skipped ===
    
            # splitting mask
            g.edata['train_mask'] = train_mask
            g.edata['val_mask'] = val_mask
            g.edata['test_mask'] = test_mask
            # edge type
            g.edata['etype'] = etype
            # node type
            g.ndata['ntype'] = ntype
            self._g = g
    
        def __getitem__(self, idx):
            assert idx == 0, "This dataset has only one graph"
            return self._g
    
        def __len__(self):
            return 1

As shown in the code, it adds splitting masks into ``edata`` field of the
graph. Check `KnowledgeGraphDataset source
code <https://docs.dgl.ai/en/0.5.x/_modules/dgl/data/knowledge_graph.html#KnowledgeGraphDataset>`__
to see the complete code. The following code uses a subclass of ``KnowledgeGraphDataset``,
:class:`dgl.data.FB15k237Dataset`, to show the usage of it:

.. code:: 

    from dgl.data import FB15k237Dataset

    # load data
    dataset = FB15k237Dataset()
    graph = dataset[0]
    
    # get training mask
    train_mask = graph.edata['train_mask']
    train_idx = torch.nonzero(train_mask, as_tuple=False).squeeze()
    src, dst = graph.edges(train_idx)
    # get edge types in training set
    rel = graph.edata['etype'][train_idx]


A complete guide for training link prediction models can be found in
:ref:`guide-training-link-prediction`.

For more examples of link prediction datasets, please refer to DGL's
builtin datasets: 

* :ref:`kgdata`

* :ref:`bitcoinotcdata`
