[bugfix] fix default ntypes/etypes consistency between dgl.DGLGraph and dgl.graph (#3198)

python/dgl/heterograph.py

@@ -39,8 +39,8 @@ class DGLHeteroGraph(object):
     # pylint: disable=unused-argument, dangerous-default-value
     def __init__(self,
                  gidx=[],
-                 ntypes=['_U'],
-                 etypes=['_V'],
+                 ntypes=['_N'],
+                 etypes=['_E'],
                  node_frames=None,
                  edge_frames=None,
                  **deprecate_kwargs):

tests/compute/test_graph.py

@@ -369,6 +369,13 @@ def test_is_sorted():
    assert src_sorted == True
    assert dst_sorted == False
 
+
+def test_default_types():
+    dg = dgl.DGLGraph()
+    g = dgl.graph(([], []))
+    assert dg.ntypes == g.ntypes
+    assert dg.etypes == g.etypes
+
 if __name__ == '__main__':
     test_query()
     test_mutation()
@@ -377,3 +384,4 @@ if __name__ == '__main__':
     test_find_edges()
     test_hypersparse_query()
     test_is_sorted()
+    test_default_types()