Fix binding to a single stateful actuator.

When binding single elements, the named_index argument to Binding is a string, rather than a list of strings. Most code in the Binding class handles this, but for stateful actuators, _filter_stateful_actuators didn't handle the case of the single actuator name, which caused a crash.

PiperOrigin-RevId: 450437063
Change-Id: I7e47287d162fb2c765a013d86fb104f09caebd64

Author: nimrod-gileadi

dm_control/mjcf/physics.py

@@ -106,6 +106,9 @@ def _get_actuator_state_fields():
 
 def _filter_stateful_actuators(physics, actuator_names):
   """Removes any stateless actuators from the list of actuator names."""
+  if isinstance(actuator_names, str):
+    actuator_names = [actuator_names]
+
   if physics.model.na:
     # MuJoCo requires that stateful actuators always come after stateless
     # actuators in the model, so we keep actuator names only if their

dm_control/mjcf/physics_test.py

@@ -66,7 +66,9 @@ def test_construct_and_reload_from_mjcf_model(self):
       ('geom', True),
       ('geom', False),
       ('joint', True),
-      ('joint', False))
+      ('joint', False),
+      ('actuator', True),
+      ('actuator', False))
   def test_id(self, namespace, single_element):
     elements, full_identifiers = self.sample_elements(namespace, single_element)
     actual = self.physics.bind(elements).element_id
@@ -204,6 +206,22 @@ def test_bind_worldbody(self):
     mass = physics.bind(model.worldbody).subtreemass
     self.assertEqual(mass, expected_mass)
 
+  def test_bind_stateful_actuator(self):
+    model = mjcf.RootElement()
+    body = model.worldbody.add('body')
+    body.add('joint', name='joint')
+    body.add('geom', type='sphere', size=[1])
+
+    model.actuator.add(
+        'general', name='act1', joint='joint', dyntype='integrator')
+
+    physics = mjcf.Physics.from_mjcf_model(model)
+    actuator = model.find('actuator', 'act1')
+    binding = physics.bind(actuator)
+
+    # This used to crash
+    self.assertEqual(0, binding.act)
+
   def test_caching(self):
     all_joints = self.model.find_all('joint')