Add 's' and 'z' variable support to tf() (#304)

murrayrm · web-flow · commit 8a11cd3688bd · 2019-05-31T13:11:51.000-04:00
diff --git a/control/tests/matlab_test.py b/control/tests/matlab_test.py
@@ -664,6 +664,22 @@ def testCombi01(self):
         self.assertAlmostEqual(wg, 0.176469728448)
         self.assertAlmostEqual(wp, 0.0616288455466)
 
+    def test_tf_string_args(self):
+        # Make sure that the 's' variable is defined properly
+        s = tf('s')
+        G = (s + 1)/(s**2 + 2*s + 1)
+        np.testing.assert_array_almost_equal(G.num, [[[1, 1]]])
+        np.testing.assert_array_almost_equal(G.den, [[[1, 2, 1]]])
+        self.assertTrue(isctime(G, strict=True))
+
+        # Make sure that the 'z' variable is defined properly
+        z = tf('z')
+        G = (z + 1)/(z**2 + 2*z + 1)
+        np.testing.assert_array_almost_equal(G.num, [[[1, 1]]])
+        np.testing.assert_array_almost_equal(G.den, [[[1, 2, 1]]])
+        self.assertTrue(isdtime(G, strict=True))
+
+
 #! TODO: not yet implemented
 #    def testMIMOtfdata(self):
 #        sisotf = ss2tf(self.siso_ss1)
diff --git a/control/tests/xferfcn_test.py b/control/tests/xferfcn_test.py
@@ -11,8 +11,8 @@
     ss2tf
 from control.lti import evalfr
 from control.exception import slycot_check
+from control.lti import isctime, isdtime
 from control.dtime import sample_system
-# from control.lti import isdtime
 
 
 class TestXferFcn(unittest.TestCase):
@@ -704,6 +704,21 @@ def test_ss2tf(self):
         np.testing.assert_almost_equal(sys.num, true_sys.num)
         np.testing.assert_almost_equal(sys.den, true_sys.den)
 
+    def test_class_constants(self):
+        # Make sure that the 's' variable is defined properly
+        s = TransferFunction.s
+        G = (s + 1)/(s**2 + 2*s + 1)
+        np.testing.assert_array_almost_equal(G.num, [[[1, 1]]])
+        np.testing.assert_array_almost_equal(G.den, [[[1, 2, 1]]])
+        self.assertTrue(isctime(G, strict=True))
+
+        # Make sure that the 'z' variable is defined properly
+        z = TransferFunction.z
+        G = (z + 1)/(z**2 + 2*z + 1)
+        np.testing.assert_array_almost_equal(G.num, [[[1, 1]]])
+        np.testing.assert_array_almost_equal(G.den, [[[1, 2, 1]]])
+        self.assertTrue(isdtime(G, strict=True))
+
     def test_printing(self):
         # SISO, continuous time
         sys = ss2tf(rss(4, 1, 1))
diff --git a/control/xferfcn.py b/control/xferfcn.py
