dgl.node_subgraph¶

dgl.node_subgraph(graph, nodes)[source]¶

Return a subgraph induced on the given nodes.

A node-induced subgraph is a subset of the nodes of a graph together with any edges whose endpoints are both in this subset. In addition to extracting the subgraph, DGL conducts the following:

Relabel the extracted nodes to IDs starting from zero.
Copy the features of the extracted nodes and edges to the resulting graph. The copy is lazy and incurs data movement only when needed.
Store the IDs of the extracted nodes and edges in the ndata and edata of the resulting graph under name dgl.NID and dgl.EID, respectively.

If the graph is heterogeneous, DGL extracts a subgraph per relation and composes them as the resulting graph. Thus, the resulting graph has the same set of relations as the input one.

Parameters

graph (DGLGraph) – The graph to extract subgraphs from.
nodes (nodes or dict[str, nodes]) –
The nodes to form the subgraph. The allowed nodes formats are:
- Int Tensor: Each element is a node ID. The tensor must have the same device type and ID data type as the graph’s.
- iterable[int]: Each element is a node ID.
- Bool Tensor: Each \(i^{th}\) element is a bool flag indicating whether node \(i\) is in the subgraph.
If the graph is homogeneous, one can directly pass the above formats. Otherwise, the argument must be a dictionary with keys being node types and values being the nodes.

Returns

G – The subgraph.

Return type

DGLGraph

Examples

The following example uses PyTorch backend.

>>> import dgl
>>> import torch

Extract a subgraph from a homogeneous graph.

>>> g = dgl.graph(([0, 1, 2, 3, 4], [1, 2, 3, 4, 0]))  # 5-node cycle
>>> sg = dgl.node_subgraph(g, [0, 1, 4])
>>> sg
Graph(num_nodes=3, num_edges=2,
      ndata_schemes={'_ID': Scheme(shape=(), dtype=torch.int64)}
      edata_schemes={'_ID': Scheme(shape=(), dtype=torch.int64)})
>>> sg.edges()
(tensor([0, 2]), tensor([1, 0]))
>>> sg.ndata[dgl.NID]  # original node IDs
tensor([0, 1, 4])
>>> sg.edata[dgl.EID]  # original edge IDs
tensor([0, 4])

Specify nodes using a boolean mask.

>>> nodes = torch.tensor([True, True, False, False, True])  # choose nodes [0, 1, 4]
>>> dgl.node_subgraph(g, nodes)
Graph(num_nodes=3, num_edges=2,
      ndata_schemes={'_ID': Scheme(shape=(), dtype=torch.int64)}
      edata_schemes={'_ID': Scheme(shape=(), dtype=torch.int64)})

The resulting subgraph also copies features from the parent graph.

>>> g.ndata['x'] = torch.arange(10).view(5, 2)
>>> sg = dgl.node_subgraph(g, [0, 1, 4])
>>> sg
Graph(num_nodes=3, num_edges=2,
      ndata_schemes={'x': Scheme(shape=(2,), dtype=torch.int64),
                     '_ID': Scheme(shape=(), dtype=torch.int64)}
      edata_schemes={'_ID': Scheme(shape=(), dtype=torch.int64)})
>>> sg.ndata['x']
tensor([[0, 1],
        [2, 3],
        [8, 9]])

Extract a subgraph from a hetergeneous graph.

>>> g = dgl.heterograph({
>>>     ('user', 'plays', 'game'): ([0, 1, 1, 2], [0, 0, 2, 1]),
>>>     ('user', 'follows', 'user'): ([0, 1, 1], [1, 2, 2])
>>> })
>>> sub_g = dgl.node_subgraph(g, {'user': [1, 2]})
>>> sub_g
Graph(num_nodes={'user': 2, 'game': 0},
      num_edges={('user', 'plays', 'game'): 0, ('user', 'follows', 'user'): 2},
      metagraph=[('user', 'game'), ('user', 'user')])