[BUG] Extract subgraph on pg creates an undirected graph

[VibhuJawa]

Describe the bug

Extract subgraph on a PG creates an undirected graph instead of directed one

Steps/Code to reproduce bug

import cugraph
import cudf
from cugraph.experimental import PropertyGraph

# create PG
df = cudf.DataFrame({'src':[1,2], 'dst':[2,3]})
pg = PropertyGraph()
pg.add_edge_data(df, vertex_col_names=['src','dst'])

# extract subgraph
graph = pg.extract_subgraph()

print(graph.directed)

False

print(graph.edges())

   src  dst
0    2    3
1    1    2

Expected behavior

I would expect a directed graph.

Additional context
This means that i cant use this extracted subgraph for sampling in GNN pipe-lines.

CC: @rlratzel , @alexbarghi-nv

[eriknw]

Try graph = pg.extract_subgraph(cugraph.DiGraph). The first argument to extract_subgraph is create_using=cugraph.Graph.

Do you think the default should be cugraph.DiGraph?

[rlratzel]

Try using the optional create_using arg:

graph = pg.extract_subgraph(create_using=cugraph.Graph(directed=True))

create_using accepts either a type or an instance, and uses it when creating the subgraph to return. When passing an instance, it will copy all the graph attributes (in this case, directed) when creating the new instance to return. This allows you to use any existing Graph object as a "template" when extracting subgraphs (eg. "I want a subgraph of the same type with the same options as G").

I have a test of extract_subgraph for directed graphs here as another example.

You can access the docstring of extract_subgraph using the interactive help to see all other available args:

>>> help(pg.extract_subgraph)

[rlratzel]

(oops, Erik beat me to it)

[rlratzel]

Try graph = pg.extract_subgraph(cugraph.DiGraph). The first argument to extract_subgraph is create_using=cugraph.Graph.

FYI, cugraph.DiGraph is deprecated in favor of the directed=True arg to the cugraph.Graph() ctor.

Do you think the default should be cugraph.DiGraph?

This is a good question, IOW, should we default to returning directed graphs if create_using isn't specified?

[VibhuJawa]

Closing in favor of #2459 . Thanks guys .