Add convenient plugin assignment in Python bindings.

PiperOrigin-RevId: 861168536
Change-Id: I274fbf3460d475434f837ab58bf5e2b0988369fd

python/mujoco/codegen/generate_spec_bindings.py

@@ -93,16 +93,26 @@ def _value_binding_code(
   fulltype = fulltype.replace('mjOption', 'raw::MjOption')
   fulltype = fulltype.replace('mjVisual', 'raw::MjVisual')
   fulltype = fulltype.replace('mjStatistic', 'raw::MjStatistic')
-  element = '.element' if fullvarname == 'plugin' else ''
+  element = ''
+
+  if field.name == 'mjsPlugin':
+    setter = f"""[]({rawclassname}& self, {fulltype} {varname}) {{
+      if (self.{fullvarname}.name && {varname}.name) *self.{fullvarname}.name = *{varname}.name;
+      if (self.{fullvarname}.plugin_name && {varname}.plugin_name) *self.{fullvarname}.plugin_name = *{varname}.plugin_name;
+      self.{fullvarname}.active = {varname}.active;
+      if (self.{fullvarname}.info && {varname}.info) *self.{fullvarname}.info = *{varname}.info;
+    }}"""
+  else:
+    setter = f"""[]({rawclassname}& self, {fulltype} {varname}) {{
+      self.{fullvarname}{element} = {varname}{element};
+    }}"""
 
   def_property_args = (
       f'"{varname}"',
       f"""[]({rawclassname}& self) -> {fulltype} {{
         return self.{fullvarname};
       }}""",
-      f"""[]({rawclassname}& self, {fulltype} {varname}) {{
-        self.{fullvarname}{element} = {varname}{element};
-      }}""",
+      setter,
   )
 
   if field.name not in SCALAR_TYPES:

python/mujoco/specs_test.py

@@ -996,6 +996,23 @@ class SpecsTest(absltest.TestCase):
     self.assertIsNotNone(model)
     self.assertEqual(model.nplugin, 0)
 
+  def testPluginAssignment(self):
+    spec = mujoco.MjSpec()
+    body = spec.worldbody.add_body()
+    body.name = 'test_body'
+
+    plugin = spec.add_plugin(
+        name='test_instance', plugin_name='mujoco.elasticity.cable', active=True
+    )
+    plugin.config = {'twist': '1e2', 'bend': '4e1'}
+
+    # Assignment should copy active flag and names
+    body.plugin = plugin
+
+    self.assertTrue(body.plugin.active)
+    self.assertEqual(body.plugin.name, 'test_instance')
+    self.assertEqual(body.plugin.plugin_name, 'mujoco.elasticity.cable')
+
   def test_access_option_stat_visual(self):
     spec = mujoco.MjSpec.from_string("""
       <mujoco model="MuJoCo Model">