[BUGFIX] check the number of columns in subgraph.copy_from_parent (#903)

* check the number of columns.

* add test.

python/dgl/subgraph.py

@@ -116,10 +116,10 @@ class DGLSubGraph(DGLGraph):
 
         All old features will be removed.
         """
-        if self._parent._node_frame.num_rows != 0:
+        if self._parent._node_frame.num_rows != 0 and self._parent._node_frame.num_columns != 0:
             self._node_frame = FrameRef(Frame(
                 self._parent._node_frame[self._parent_nid]))
-        if self._parent._edge_frame.num_rows != 0:
+        if self._parent._edge_frame.num_rows != 0 and self._parent._edge_frame.num_columns != 0:
             self._edge_frame = FrameRef(Frame(
                 self._parent._edge_frame[self._get_parent_eid()]))
 

tests/compute/test_subgraph.py

@@ -4,7 +4,7 @@ import backend as F
 
 D = 5
 
-def generate_graph(grad=False):
+def generate_graph(grad=False, add_data=True):
     g = DGLGraph()
     g.add_nodes(10)
     # create a graph where 0 is the source and 9 is the sink
@@ -13,6 +13,7 @@ def generate_graph(grad=False):
         g.add_edge(i, 9)
     # add a back flow from 9 to 0
     g.add_edge(9, 0)
+    if add_data:
         ncol = F.randn((10, D))
         ecol = F.randn((17, D))
         if grad:
@@ -22,6 +23,15 @@ def generate_graph(grad=False):
         g.edata['l'] = ecol
     return g
 
+def test_basics1():
+    # Test when the graph has no node data and edge data.
+    g = generate_graph(add_data=False)
+    eid = [0, 2, 3, 6, 7, 9]
+    sg = g.edge_subgraph(eid)
+    sg.copy_from_parent()
+    sg.ndata['h'] = F.arange(0, sg.number_of_nodes())
+    sg.edata['h'] = F.arange(0, sg.number_of_edges())
+
 def test_basics():
     g = generate_graph()
     h = g.ndata['h']
@@ -96,4 +106,5 @@ def test_merge():
 
 if __name__ == '__main__':
     test_basics()
+    test_basics1()
     #test_merge()