Note

Go to the end to download the full example code

# Training a GNN for Graph Classificationο

By the end of this tutorial, you will be able to

Load a DGL-provided graph classification dataset.

Understand what

*readout*function does.Understand how to create and use a minibatch of graphs.

Build a GNN-based graph classification model.

Train and evaluate the model on a DGL-provided dataset.

(Time estimate: 18 minutes)

```
import os
os.environ["DGLBACKEND"] = "pytorch"
import dgl
import dgl.data
import torch
import torch.nn as nn
import torch.nn.functional as F
```

## Overview of Graph Classification with GNNο

Graph classification or regression requires a model to predict certain graph-level properties of a single graph given its node and edge features. Molecular property prediction is one particular application.

This tutorial shows how to train a graph classification model for a small dataset from the paper How Powerful Are Graph Neural Networks.

## Loading Dataο

```
# Generate a synthetic dataset with 10000 graphs, ranging from 10 to 500 nodes.
dataset = dgl.data.GINDataset("PROTEINS", self_loop=True)
```

The dataset is a set of graphs, each with node features and a single
label. One can see the node feature dimensionality and the number of
possible graph categories of `GINDataset`

objects in `dim_nfeats`

and `gclasses`

attributes.

```
print("Node feature dimensionality:", dataset.dim_nfeats)
print("Number of graph categories:", dataset.gclasses)
from dgl.dataloading import GraphDataLoader
```

```
Node feature dimensionality: 3
Number of graph categories: 2
```

## Defining Data Loaderο

A graph classification dataset usually contains two types of elements: a
set of graphs, and their graph-level labels. Similar to an image
classification task, when the dataset is large enough, we need to train
with mini-batches. When you train a model for image classification or
language modeling, you will use a `DataLoader`

to iterate over the
dataset. In DGL, you can use the `GraphDataLoader`

.

You can also use various dataset samplers provided in
torch.utils.data.sampler.
For example, this tutorial creates a training `GraphDataLoader`

and
test `GraphDataLoader`

, using `SubsetRandomSampler`

to tell PyTorch
to sample from only a subset of the dataset.

```
from torch.utils.data.sampler import SubsetRandomSampler
num_examples = len(dataset)
num_train = int(num_examples * 0.8)
train_sampler = SubsetRandomSampler(torch.arange(num_train))
test_sampler = SubsetRandomSampler(torch.arange(num_train, num_examples))
train_dataloader = GraphDataLoader(
dataset, sampler=train_sampler, batch_size=5, drop_last=False
)
test_dataloader = GraphDataLoader(
dataset, sampler=test_sampler, batch_size=5, drop_last=False
)
```

You can try to iterate over the created `GraphDataLoader`

and see what it
gives:

```
[Graph(num_nodes=138, num_edges=698,
ndata_schemes={'attr': Scheme(shape=(3,), dtype=torch.float32), 'label': Scheme(shape=(), dtype=torch.int64)}
edata_schemes={}), tensor([0, 1, 0, 0, 0])]
```

As each element in `dataset`

has a graph and a label, the
`GraphDataLoader`

will return two objects for each iteration. The
first element is the batched graph, and the second element is simply a
label vector representing the category of each graph in the mini-batch.
Next, weβll talked about the batched graph.

## A Batched Graph in DGLο

In each mini-batch, the sampled graphs are combined into a single bigger
batched graph via `dgl.batch`

. The single bigger batched graph merges
all original graphs as separately connected components, with the node
and edge features concatenated. This bigger graph is also a `DGLGraph`

instance (so you can
still treat it as a normal `DGLGraph`

object as in
here). It however contains the information
necessary for recovering the original graphs, such as the number of
nodes and edges of each graph element.

```
batched_graph, labels = batch
print(
"Number of nodes for each graph element in the batch:",
batched_graph.batch_num_nodes(),
)
print(
"Number of edges for each graph element in the batch:",
batched_graph.batch_num_edges(),
)
# Recover the original graph elements from the minibatch
graphs = dgl.unbatch(batched_graph)
print("The original graphs in the minibatch:")
print(graphs)
```

```
Number of nodes for each graph element in the batch: tensor([24, 22, 29, 30, 33])
Number of edges for each graph element in the batch: tensor([108, 110, 143, 148, 189])
The original graphs in the minibatch:
[Graph(num_nodes=24, num_edges=108,
ndata_schemes={'attr': Scheme(shape=(3,), dtype=torch.float32), 'label': Scheme(shape=(), dtype=torch.int64)}
edata_schemes={}), Graph(num_nodes=22, num_edges=110,
ndata_schemes={'attr': Scheme(shape=(3,), dtype=torch.float32), 'label': Scheme(shape=(), dtype=torch.int64)}
edata_schemes={}), Graph(num_nodes=29, num_edges=143,
ndata_schemes={'attr': Scheme(shape=(3,), dtype=torch.float32), 'label': Scheme(shape=(), dtype=torch.int64)}
edata_schemes={}), Graph(num_nodes=30, num_edges=148,
ndata_schemes={'attr': Scheme(shape=(3,), dtype=torch.float32), 'label': Scheme(shape=(), dtype=torch.int64)}
edata_schemes={}), Graph(num_nodes=33, num_edges=189,
ndata_schemes={'attr': Scheme(shape=(3,), dtype=torch.float32), 'label': Scheme(shape=(), dtype=torch.int64)}
edata_schemes={})]
```

## Define Modelο

This tutorial will build a two-layer Graph Convolutional Network (GCN). Each of its layer computes new node representations by aggregating neighbor information. If you have gone through the introduction, you will notice two differences:

Since the task is to predict a single category for the

*entire graph*instead of for every node, you will need to aggregate the representations of all the nodes and potentially the edges to form a graph-level representation. Such process is more commonly referred as a*readout*. A simple choice is to average the node features of a graph with`dgl.mean_nodes()`

.The input graph to the model will be a batched graph yielded by the

`GraphDataLoader`

. The readout functions provided by DGL can handle batched graphs so that they will return one representation for each minibatch element.

```
from dgl.nn import GraphConv
class GCN(nn.Module):
def __init__(self, in_feats, h_feats, num_classes):
super(GCN, self).__init__()
self.conv1 = GraphConv(in_feats, h_feats)
self.conv2 = GraphConv(h_feats, num_classes)
def forward(self, g, in_feat):
h = self.conv1(g, in_feat)
h = F.relu(h)
h = self.conv2(g, h)
g.ndata["h"] = h
return dgl.mean_nodes(g, "h")
```

## Training Loopο

The training loop iterates over the training set with the
`GraphDataLoader`

object and computes the gradients, just like
image classification or language modeling.

```
# Create the model with given dimensions
model = GCN(dataset.dim_nfeats, 16, dataset.gclasses)
optimizer = torch.optim.Adam(model.parameters(), lr=0.01)
for epoch in range(20):
for batched_graph, labels in train_dataloader:
pred = model(batched_graph, batched_graph.ndata["attr"].float())
loss = F.cross_entropy(pred, labels)
optimizer.zero_grad()
loss.backward()
optimizer.step()
num_correct = 0
num_tests = 0
for batched_graph, labels in test_dataloader:
pred = model(batched_graph, batched_graph.ndata["attr"].float())
num_correct += (pred.argmax(1) == labels).sum().item()
num_tests += len(labels)
print("Test accuracy:", num_correct / num_tests)
```

```
Test accuracy: 0.27802690582959644
```

## Whatβs nextο

See GIN example for an end-to-end graph classification model.

```
# Thumbnail credits: DGL
# sphinx_gallery_thumbnail_path = '_static/blitz_5_graph_classification.png'
```

**Total running time of the script:** (0 minutes 28.965 seconds)