Is there any support for batching Heterogenous graphs? If not, any tips as to the best approach to implement it myself?

# Batching Heterogenous graphs

I will work on this, but will need some more time. For the time being, I’d suggest you rather explicitly construct multiple homographs and batching each type of graphs separately.

Thanks! We thought of this hack, but its not so straight-forward when there are edges between different node types…

@lederg I was looking for batching heterograph and get a workaround with the code block below that you may find useful. I did not test the speed, but I think it would not be too bad, at least better than looping over individual graphs.

Note that the below code only works for nodes and their associated features. No edge features and the order of edges may change in the batched graph.

```
def graph_list_to_batch(graph_list):
# query graph info
g = graph_list[0]
ntypes = g.ntypes
etypes = g.canonical_etypes
attrs = {t: g.nodes[t].data.keys() for t in ntypes}
# graph connectivity
current_num_nodes = {t: 0 for t in ntypes}
connectivity = {t: [] for t in etypes}
for g in graph_list:
for t in etypes:
src, edge, dest = t
conn = []
for i in range(g.number_of_nodes(src)):
i_prime = i + current_num_nodes[src]
conn.extend(
[
(i_prime, j + current_num_nodes[dest])
for j in g.successors(i, edge)
]
)
connectivity[t].extend(conn)
for t in ntypes:
current_num_nodes[t] += g.number_of_nodes(t)
# create batched graph
batch_g = dgl.heterograph(connectivity)
# graph data (node only)
slices = {n: [] for n in ntypes}
data = {n: defaultdict(list) for n in ntypes}
for g in graph_list:
for t in ntypes:
for a in attrs[t]:
data[t][a].append(g.nodes[t].data[a])
slices[t].append(g.number_of_nodes(t))
# batch data
for t, dt in data.items():
for a, d in dt.items():
data[t][a] = torch.cat(d)
# add batch data to batch graph
for t in ntypes:
batch_g.nodes[t].data.update(data[t])
# attach graph list and data slices for later split
batch_g.graph_list = graph_list
batch_g.node_slices = slices
return batch_g
```

@lederg @mjwen The support for batching/unbatching DGLHeteroGraph has been added and you can use it by installing from source. See the examples. Let me know if you have any questions.

is there a way to support batching hetero graph with different ntypes and etypes? or is there a way to create hetero graph that can have place holder for ntypes and etypes that does not exist so that they can be batched together ?

`dgl.batch_hetero`

takes a list of HeteroDGLGraph `g_list`

and you can directly construct a list of sliced HeteroDGLGraph with `[g[etype] for g in g_list]`

.