improve and test is_directed

pygsp/graphs/graph.py

@@ -395,7 +395,7 @@ def is_directed(self, recompute=False):
         r"""Check if the graph has directed edges (cached).
 
         In this framework, we consider that a graph is directed if and
-        only if its weight matrix is non symmetric.
+        only if its weight matrix is not symmetric.
 
         Parameters
         ----------
@@ -407,10 +407,6 @@ def is_directed(self, recompute=False):
         directed : bool
             True if the graph is directed.
 
-        Notes
-        -----
-        Can also be used to check if a matrix is symmetrical
-
         Examples
         --------
 
@@ -440,7 +436,7 @@ def is_directed(self, recompute=False):
         if hasattr(self, '_directed') and not recompute:
             return self._directed
 
-        self._directed = np.abs(self.W - self.W.T).sum() != 0
+        self._directed = (self.W != self.W.T).nnz != 0
         return self._directed
 
     def extract_components(self):

pygsp/tests/test_graphs.py

@@ -85,6 +85,22 @@ def test_is_connected(self):
         self.assertEqual(graph.is_directed(), True)
         self.assertEqual(graph.is_connected(), True)
 
+    def test_is_directed(self):
+        graph = graphs.Graph([
+            [0, 3, 0, 0],
+            [3, 0, 4, 0],
+            [0, 4, 0, 2],
+            [0, 0, 2, 0],
+        ])
+        assert graph.W.nnz == 6
+        self.assertEqual(graph.is_directed(), False)
+        graph.W[0, 1] = 0
+        assert graph.W.nnz == 6
+        self.assertEqual(graph.is_directed(recompute=True), True)
+        graph.W[1, 0] = 0
+        assert graph.W.nnz == 6
+        self.assertEqual(graph.is_directed(recompute=True), False)
+
     def test_laplacian(self):
 
         adjacency = np.array([