[BUG] Another fix on remove edges when all edges are removed (#1386)

* [BUG] Another fix on remove edges when all edges are removed

* fix mxnet

python/dgl/backend/mxnet/tensor.py

@@ -335,6 +335,9 @@ def sort_1d(input):
     return val, idx
 
 def arange(start, stop):
+    if start >= stop:
+        return nd.array([], dtype=np.int64)
+    else:
         return nd.arange(start, stop, dtype=np.int64)
 
 def rand_shuffle(arr):

python/dgl/transform.py

@@ -931,9 +931,12 @@ def remove_edges(g, edge_ids):
     for i, canonical_etype in enumerate(g.canonical_etypes):
         data = induced_eids_nd[i].data
         if len(data) == 0:
-            # Empty means that no edges are removed and edges are not shuffled.
+            # Empty means that either
+            # (1) no edges are removed and edges are not shuffled.
+            # (2) all edges are removed.
+            # The following statement deals with both cases.
             new_graph.edges[canonical_etype].data[EID] = F.arange(
-                0, g.number_of_edges(canonical_etype))
+                0, new_graph.number_of_edges(canonical_etype))
         else:
             new_graph.edges[canonical_etype].data[EID] = F.zerocopy_from_dgl_ndarray(data)
 

tests/compute/test_transform.py

@@ -547,9 +547,9 @@ def test_remove_edges():
     check(g3, 'AB', g, [3])
     check(g3, 'BA', g, [1])
 
-    g4 = dgl.remove_edges(g, {'AB': F.tensor([3])})
+    g4 = dgl.remove_edges(g, {'AB': F.tensor([3, 1, 2, 0])})
     check(g4, 'AA', g, [])
-    check(g4, 'AB', g, [3])
+    check(g4, 'AB', g, [3, 1, 2, 0])
     check(g4, 'BA', g, [])
 
 if __name__ == '__main__':