fix bug when getting edge list of empty graph

pygsp/graphs/graph.py

@@ -702,11 +702,14 @@ def get_edge_list(self):
 
         """
 
-        W = self.W if self.is_directed() else sparse.tril(self.W)
+        if self.is_directed():
+            W = self.W.tocoo()
+        else:
+            W = sparse.tril(self.W, format='coo')
 
-        sources, targets = W.nonzero()
-        weights = self.W[sources, targets]
-        weights = np.asarray(weights).squeeze()
+        sources = W.row
+        targets = W.col
+        weights = W.data
 
         assert self.n_edges == sources.size == targets.size == weights.size
         return sources, targets, weights

pygsp/tests/test_graphs.py

@@ -166,6 +166,21 @@ def test_difference(self):
             self.assertEqual(len(Ls), G.n_vertices)
             np.testing.assert_allclose(Ls, G.L.dot(self._signal))
 
+    def test_empty_graph(self, n_vertices=11):
+        graph = graphs.Graph(np.zeros((n_vertices, n_vertices)))
+        self.assertEqual(graph.n_vertices, n_vertices)
+        self.assertEqual(graph.n_edges, 0)
+        self.assertEqual(graph.W.nnz, 0)
+        self.assertEqual(graph.L.nnz, 0)
+        sources, targets, weights = graph.get_edge_list()
+        self.assertEqual(len(sources), 0)
+        self.assertEqual(len(targets), 0)
+        self.assertEqual(len(weights), 0)
+        graph.compute_differential_operator()
+        self.assertEqual(graph.D.nnz, 0)
+        graph.compute_fourier_basis()
+        np.testing.assert_allclose(graph.U, np.identity(n_vertices))
+
     def test_set_coordinates(self):
         G = graphs.FullConnected()
         coords = self._rs.uniform(size=(G.N, 2))