use standard method to assess whether a system is a static gain, so that it has dt=None by default, when constructing a LinearIOSystem

sawyerbfuller · sawyerbfuller · commit 588e04600af2 · 2022-11-07T20:23:23.000-08:00
diff --git a/control/statesp.py b/control/statesp.py
@@ -346,9 +346,8 @@ def __init__(self, *args, init_namedio=True, **kwargs):
             defaults = args[0] if len(args) == 1 else \
                 {'inputs': D.shape[1], 'outputs': D.shape[0],
                  'states': A.shape[0]}
-            static = (A.size == 0)
             name, inputs, outputs, states, dt = _process_namedio_keywords(
-                kwargs, defaults, static=static, end=True)
+                kwargs, defaults, static=self._isstatic(), end=True)
 
             # Initialize LTI (NamedIOSystem) object
             super().__init__(
diff --git a/control/tests/iosys_test.py b/control/tests/iosys_test.py
@@ -41,6 +41,9 @@ class TSys:
             [[-1, 1], [0, -2]], [[0, 1], [1, 0]],
             [[1, 0], [0, 1]], np.zeros((2, 2)))
 
+        # Create a static gain linear system
+        T.staticgain = ct.StateSpace(0, 0, 0, 1)
+
         # Create simulation parameters
         T.T = np.linspace(0, 10, 100)
         T.U = np.sin(T.T)
@@ -70,6 +73,12 @@ def test_linear_iosys(self, tsys):
         # system results in a LinearIOSystem
         assert isinstance(linsys*iosys, ios.LinearIOSystem) 
 
+        # Make sure that a static linear system has dt=None 
+        # and otherwise dt is as specified
+        assert ios.LinearIOSystem(tsys.staticgain).dt is None
+        assert ios.LinearIOSystem(tsys.staticgain, dt=.1).dt == .1
+
+
     def test_tf2io(self, tsys):
         # Create a transfer function from the state space system
         linsys = tsys.siso_linsys
