Add non-tree element lists to python bindings.

Fixes #1752.

PiperOrigin-RevId: 649405039
Change-Id: Ie0b575a3ec912b773141694397ac2f394bff1227

doc/APIreference/functions.rst

@@ -4176,6 +4176,24 @@ mjs_nextChild
 
 Return body's next child of the same type; return NULL if child is last.
 
+.. _mjs_firstElement:
+
+mjs_firstElement
+~~~~~~~~~~~~~~~~
+
+.. mujoco-include:: mjs_firstElement
+
+Return spec's first element of selected type.
+
+.. _mjs_nextElement:
+
+mjs_nextElement
+~~~~~~~~~~~~~~~
+
+.. mujoco-include:: mjs_nextElement
+
+Return spec's next element; return NULL if element is last.
+
 .. _mjs_asBody:
 
 mjs_asBody
@@ -4239,6 +4257,150 @@ mjs_asFrame
 
 Safely cast an element as mjsFrame, or return NULL if the element is not an mjsFrame.
 
+.. _mjs_asActuator:
+
+mjs_asActuator
+~~~~~~~~~~~~~~
+
+.. mujoco-include:: mjs_asActuator
+
+Safely cast an element as mjsActuator, or return NULL if the element is not an mjsActuator.
+
+.. _mjs_asSensor:
+
+mjs_asSensor
+~~~~~~~~~~~~
+
+.. mujoco-include:: mjs_asSensor
+
+Safely cast an element as mjsSensor, or return NULL if the element is not an mjsSensor.
+
+.. _mjs_asFlex:
+
+mjs_asFlex
+~~~~~~~~~~
+
+.. mujoco-include:: mjs_asFlex
+
+Safely cast an element as mjsFlex, or return NULL if the element is not an mjsFlex.
+
+.. _mjs_asPair:
+
+mjs_asPair
+~~~~~~~~~~
+
+.. mujoco-include:: mjs_asPair
+
+Safely cast an element as mjsPair, or return NULL if the element is not an mjsPair.
+
+.. _mjs_asEquality:
+
+mjs_asEquality
+~~~~~~~~~~~~~~
+
+.. mujoco-include:: mjs_asEquality
+
+Safely cast an element as mjsEquality, or return NULL if the element is not an mjsEquality.
+
+.. _mjs_asExclude:
+
+mjs_asExclude
+~~~~~~~~~~~~~
+
+.. mujoco-include:: mjs_asExclude
+
+Safely cast an element as mjsExclude, or return NULL if the element is not an mjsExclude.
+
+.. _mjs_asTendon:
+
+mjs_asTendon
+~~~~~~~~~~~~
+
+.. mujoco-include:: mjs_asTendon
+
+Safely cast an element as mjsTendon, or return NULL if the element is not an mjsTendon.
+
+.. _mjs_asNumeric:
+
+mjs_asNumeric
+~~~~~~~~~~~~~
+
+.. mujoco-include:: mjs_asNumeric
+
+Safely cast an element as mjsNumeric, or return NULL if the element is not an mjsNumeric.
+
+.. _mjs_asText:
+
+mjs_asText
+~~~~~~~~~~
+
+.. mujoco-include:: mjs_asText
+
+Safely cast an element as mjsText, or return NULL if the element is not an mjsText.
+
+.. _mjs_asTuple:
+
+mjs_asTuple
+~~~~~~~~~~~
+
+.. mujoco-include:: mjs_asTuple
+
+Safely cast an element as mjsTuple, or return NULL if the element is not an mjsTuple.
+
+.. _mjs_asKey:
+
+mjs_asKey
+~~~~~~~~~
+
+.. mujoco-include:: mjs_asKey
+
+Safely cast an element as mjsKey, or return NULL if the element is not an mjsKey.
+
+.. _mjs_asMesh:
+
+mjs_asMesh
+~~~~~~~~~~
+
+.. mujoco-include:: mjs_asMesh
+
+Safely cast an element as mjsMesh, or return NULL if the element is not an mjsMesh.
+
+.. _mjs_asHField:
+
+mjs_asHField
+~~~~~~~~~~~~
+
+.. mujoco-include:: mjs_asHField
+
+Safely cast an element as mjsHField, or return NULL if the element is not an mjsHField.
+
+.. _mjs_asSkin:
+
+mjs_asSkin
+~~~~~~~~~~
+
+.. mujoco-include:: mjs_asSkin
+
+Safely cast an element as mjsSkin, or return NULL if the element is not an mjsSkin.
+
+.. _mjs_asTexture:
+
+mjs_asTexture
+~~~~~~~~~~~~~
+
+.. mujoco-include:: mjs_asTexture
+
+Safely cast an element as mjsTexture, or return NULL if the element is not an mjsTexture.
+
+.. _mjs_asMaterial:
+
+mjs_asMaterial
+~~~~~~~~~~~~~~
+
+.. mujoco-include:: mjs_asMaterial
+
+Safely cast an element as mjsMaterial, or return NULL if the element is not an mjsMaterial.
+
 .. _AttributeSetters:
 
 Attribute setters

doc/includes/references.h

@@ -3551,6 +3551,8 @@ mjsDefault* mjs_getSpecDefault(mjSpec* s);
 int mjs_getId(mjsElement* element);
 mjsElement* mjs_firstChild(mjsBody* body, mjtObj type);
 mjsElement* mjs_nextChild(mjsBody* body, mjsElement* child);
+mjsElement* mjs_firstElement(mjSpec* s, mjtObj type);
+mjsElement* mjs_nextElement(mjSpec* s, mjsElement* element);
 mjsBody* mjs_asBody(mjsElement* element);
 mjsGeom* mjs_asGeom(mjsElement* element);
 mjsJoint* mjs_asJoint(mjsElement* element);
@@ -3558,6 +3560,22 @@ mjsSite* mjs_asSite(mjsElement* element);
 mjsCamera* mjs_asCamera(mjsElement* element);
 mjsLight* mjs_asLight(mjsElement* element);
 mjsFrame* mjs_asFrame(mjsElement* element);
+mjsActuator* mjs_asActuator(mjsElement* element);
+mjsSensor* mjs_asSensor(mjsElement* element);
+mjsFlex* mjs_asFlex(mjsElement* element);
+mjsPair* mjs_asPair(mjsElement* element);
+mjsEquality* mjs_asEquality(mjsElement* element);
+mjsExclude* mjs_asExclude(mjsElement* element);
+mjsTendon* mjs_asTendon(mjsElement* element);
+mjsNumeric* mjs_asNumeric(mjsElement* element);
+mjsText* mjs_asText(mjsElement* element);
+mjsTuple* mjs_asTuple(mjsElement* element);
+mjsKey* mjs_asKey(mjsElement* element);
+mjsMesh* mjs_asMesh(mjsElement* element);
+mjsHField* mjs_asHField(mjsElement* element);
+mjsSkin* mjs_asSkin(mjsElement* element);
+mjsTexture* mjs_asTexture(mjsElement* element);
+mjsMaterial* mjs_asMaterial(mjsElement* element);
 void mjs_setString(mjString* dest, const char* text);
 void mjs_setStringVec(mjStringVec* dest, const char* text);
 mjtByte mjs_setInStringVec(mjStringVec* dest, int i, const char* text);

include/mujoco/mujoco.h

@@ -1551,6 +1551,12 @@ MJAPI mjsElement* mjs_firstChild(mjsBody* body, mjtObj type);
 // Return body's next child of the same type; return NULL if child is last.
 MJAPI mjsElement* mjs_nextChild(mjsBody* body, mjsElement* child);
 
+// Return spec's first element of selected type.
+MJAPI mjsElement* mjs_firstElement(mjSpec* s, mjtObj type);
+
+// Return spec's next element; return NULL if element is last.
+MJAPI mjsElement* mjs_nextElement(mjSpec* s, mjsElement* element);
+
 // Safely cast an element as mjsBody, or return NULL if the element is not an mjsBody.
 MJAPI mjsBody* mjs_asBody(mjsElement* element);
 
@@ -1572,6 +1578,54 @@ MJAPI mjsLight* mjs_asLight(mjsElement* element);
 // Safely cast an element as mjsFrame, or return NULL if the element is not an mjsFrame.
 MJAPI mjsFrame* mjs_asFrame(mjsElement* element);
 
+// Safely cast an element as mjsActuator, or return NULL if the element is not an mjsActuator.
+MJAPI mjsActuator* mjs_asActuator(mjsElement* element);
+
+// Safely cast an element as mjsSensor, or return NULL if the element is not an mjsSensor.
+MJAPI mjsSensor* mjs_asSensor(mjsElement* element);
+
+// Safely cast an element as mjsFlex, or return NULL if the element is not an mjsFlex.
+MJAPI mjsFlex* mjs_asFlex(mjsElement* element);
+
+// Safely cast an element as mjsPair, or return NULL if the element is not an mjsPair.
+MJAPI mjsPair* mjs_asPair(mjsElement* element);
+
+// Safely cast an element as mjsEquality, or return NULL if the element is not an mjsEquality.
+MJAPI mjsEquality* mjs_asEquality(mjsElement* element);
+
+// Safely cast an element as mjsExclude, or return NULL if the element is not an mjsExclude.
+MJAPI mjsExclude* mjs_asExclude(mjsElement* element);
+
+// Safely cast an element as mjsTendon, or return NULL if the element is not an mjsTendon.
+MJAPI mjsTendon* mjs_asTendon(mjsElement* element);
+
+// Safely cast an element as mjsNumeric, or return NULL if the element is not an mjsNumeric.
+MJAPI mjsNumeric* mjs_asNumeric(mjsElement* element);
+
+// Safely cast an element as mjsText, or return NULL if the element is not an mjsText.
+MJAPI mjsText* mjs_asText(mjsElement* element);
+
+// Safely cast an element as mjsTuple, or return NULL if the element is not an mjsTuple.
+MJAPI mjsTuple* mjs_asTuple(mjsElement* element);
+
+// Safely cast an element as mjsKey, or return NULL if the element is not an mjsKey.
+MJAPI mjsKey* mjs_asKey(mjsElement* element);
+
+// Safely cast an element as mjsMesh, or return NULL if the element is not an mjsMesh.
+MJAPI mjsMesh* mjs_asMesh(mjsElement* element);
+
+// Safely cast an element as mjsHField, or return NULL if the element is not an mjsHField.
+MJAPI mjsHField* mjs_asHField(mjsElement* element);
+
+// Safely cast an element as mjsSkin, or return NULL if the element is not an mjsSkin.
+MJAPI mjsSkin* mjs_asSkin(mjsElement* element);
+
+// Safely cast an element as mjsTexture, or return NULL if the element is not an mjsTexture.
+MJAPI mjsTexture* mjs_asTexture(mjsElement* element);
+
+// Safely cast an element as mjsMaterial, or return NULL if the element is not an mjsMaterial.
+MJAPI mjsMaterial* mjs_asMaterial(mjsElement* element);
+
 
 //---------------------------------- Attribute setters ---------------------------------------------
 

introspect/functions.py

@@ -9835,6 +9835,48 @@ FUNCTIONS: Mapping[str, FunctionDecl] = dict([
          ),
          doc="Return body's next child of the same type; return NULL if child is last.",  # pylint: disable=line-too-long
      )),
+    ('mjs_firstElement',
+     FunctionDecl(
+         name='mjs_firstElement',
+         return_type=PointerType(
+             inner_type=ValueType(name='mjsElement'),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='s',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjSpec'),
+                 ),
+             ),
+             FunctionParameterDecl(
+                 name='type',
+                 type=ValueType(name='mjtObj'),
+             ),
+         ),
+         doc="Return spec's first element of selected type.",
+     )),
+    ('mjs_nextElement',
+     FunctionDecl(
+         name='mjs_nextElement',
+         return_type=PointerType(
+             inner_type=ValueType(name='mjsElement'),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='s',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjSpec'),
+                 ),
+             ),
+             FunctionParameterDecl(
+                 name='element',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjsElement'),
+                 ),
+             ),
+         ),
+         doc="Return spec's next element; return NULL if element is last.",
+     )),
     ('mjs_asBody',
      FunctionDecl(
          name='mjs_asBody',
@@ -9947,6 +9989,262 @@ FUNCTIONS: Mapping[str, FunctionDecl] = dict([
          ),
          doc='Safely cast an element as mjsFrame, or return NULL if the element is not an mjsFrame.',  # pylint: disable=line-too-long
      )),
+    ('mjs_asActuator',
+     FunctionDecl(
+         name='mjs_asActuator',
+         return_type=PointerType(
+             inner_type=ValueType(name='mjsActuator'),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='element',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjsElement'),
+                 ),
+             ),
+         ),
+         doc='Safely cast an element as mjsActuator, or return NULL if the element is not an mjsActuator.',  # pylint: disable=line-too-long
+     )),
+    ('mjs_asSensor',
+     FunctionDecl(
+         name='mjs_asSensor',
+         return_type=PointerType(
+             inner_type=ValueType(name='mjsSensor'),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='element',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjsElement'),
+                 ),
+             ),
+         ),
+         doc='Safely cast an element as mjsSensor, or return NULL if the element is not an mjsSensor.',  # pylint: disable=line-too-long
+     )),
+    ('mjs_asFlex',
+     FunctionDecl(
+         name='mjs_asFlex',
+         return_type=PointerType(
+             inner_type=ValueType(name='mjsFlex'),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='element',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjsElement'),
+                 ),
+             ),
+         ),
+         doc='Safely cast an element as mjsFlex, or return NULL if the element is not an mjsFlex.',  # pylint: disable=line-too-long
+     )),
+    ('mjs_asPair',
+     FunctionDecl(
+         name='mjs_asPair',
+         return_type=PointerType(
+             inner_type=ValueType(name='mjsPair'),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='element',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjsElement'),
+                 ),
+             ),
+         ),
+         doc='Safely cast an element as mjsPair, or return NULL if the element is not an mjsPair.',  # pylint: disable=line-too-long
+     )),
+    ('mjs_asEquality',
+     FunctionDecl(
+         name='mjs_asEquality',
+         return_type=PointerType(
+             inner_type=ValueType(name='mjsEquality'),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='element',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjsElement'),
+                 ),
+             ),
+         ),
+         doc='Safely cast an element as mjsEquality, or return NULL if the element is not an mjsEquality.',  # pylint: disable=line-too-long
+     )),
+    ('mjs_asExclude',
+     FunctionDecl(
+         name='mjs_asExclude',
+         return_type=PointerType(
+             inner_type=ValueType(name='mjsExclude'),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='element',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjsElement'),
+                 ),
+             ),
+         ),
+         doc='Safely cast an element as mjsExclude, or return NULL if the element is not an mjsExclude.',  # pylint: disable=line-too-long
+     )),
+    ('mjs_asTendon',
+     FunctionDecl(
+         name='mjs_asTendon',
+         return_type=PointerType(
+             inner_type=ValueType(name='mjsTendon'),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='element',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjsElement'),
+                 ),
+             ),
+         ),
+         doc='Safely cast an element as mjsTendon, or return NULL if the element is not an mjsTendon.',  # pylint: disable=line-too-long
+     )),
+    ('mjs_asNumeric',
+     FunctionDecl(
+         name='mjs_asNumeric',
+         return_type=PointerType(
+             inner_type=ValueType(name='mjsNumeric'),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='element',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjsElement'),
+                 ),
+             ),
+         ),
+         doc='Safely cast an element as mjsNumeric, or return NULL if the element is not an mjsNumeric.',  # pylint: disable=line-too-long
+     )),
+    ('mjs_asText',
+     FunctionDecl(
+         name='mjs_asText',
+         return_type=PointerType(
+             inner_type=ValueType(name='mjsText'),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='element',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjsElement'),
+                 ),
+             ),
+         ),
+         doc='Safely cast an element as mjsText, or return NULL if the element is not an mjsText.',  # pylint: disable=line-too-long
+     )),
+    ('mjs_asTuple',
+     FunctionDecl(
+         name='mjs_asTuple',
+         return_type=PointerType(
+             inner_type=ValueType(name='mjsTuple'),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='element',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjsElement'),
+                 ),
+             ),
+         ),
+         doc='Safely cast an element as mjsTuple, or return NULL if the element is not an mjsTuple.',  # pylint: disable=line-too-long
+     )),
+    ('mjs_asKey',
+     FunctionDecl(
+         name='mjs_asKey',
+         return_type=PointerType(
+             inner_type=ValueType(name='mjsKey'),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='element',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjsElement'),
+                 ),
+             ),
+         ),
+         doc='Safely cast an element as mjsKey, or return NULL if the element is not an mjsKey.',  # pylint: disable=line-too-long
+     )),
+    ('mjs_asMesh',
+     FunctionDecl(
+         name='mjs_asMesh',
+         return_type=PointerType(
+             inner_type=ValueType(name='mjsMesh'),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='element',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjsElement'),
+                 ),
+             ),
+         ),
+         doc='Safely cast an element as mjsMesh, or return NULL if the element is not an mjsMesh.',  # pylint: disable=line-too-long
+     )),
+    ('mjs_asHField',
+     FunctionDecl(
+         name='mjs_asHField',
+         return_type=PointerType(
+             inner_type=ValueType(name='mjsHField'),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='element',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjsElement'),
+                 ),
+             ),
+         ),
+         doc='Safely cast an element as mjsHField, or return NULL if the element is not an mjsHField.',  # pylint: disable=line-too-long
+     )),
+    ('mjs_asSkin',
+     FunctionDecl(
+         name='mjs_asSkin',
+         return_type=PointerType(
+             inner_type=ValueType(name='mjsSkin'),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='element',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjsElement'),
+                 ),
+             ),
+         ),
+         doc='Safely cast an element as mjsSkin, or return NULL if the element is not an mjsSkin.',  # pylint: disable=line-too-long
+     )),
+    ('mjs_asTexture',
+     FunctionDecl(
+         name='mjs_asTexture',
+         return_type=PointerType(
+             inner_type=ValueType(name='mjsTexture'),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='element',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjsElement'),
+                 ),
+             ),
+         ),
+         doc='Safely cast an element as mjsTexture, or return NULL if the element is not an mjsTexture.',  # pylint: disable=line-too-long
+     )),
+    ('mjs_asMaterial',
+     FunctionDecl(
+         name='mjs_asMaterial',
+         return_type=PointerType(
+             inner_type=ValueType(name='mjsMaterial'),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='element',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjsElement'),
+                 ),
+             ),
+         ),
+         doc='Safely cast an element as mjsMaterial, or return NULL if the element is not an mjsMaterial.',  # pylint: disable=line-too-long
+     )),
     ('mjs_setString',
      FunctionDecl(
          name='mjs_setString',

python/mujoco/specs.cc

@@ -297,6 +297,198 @@ PYBIND11_MODULE(_specs, m) {
   mjSpec.def("detach_body", [](MjSpec& self, raw::MjsBody& body) {
     mjs_detachBody(self.ptr, &body);
   });
+  mjSpec.def_property_readonly(
+    "actuators",
+    [](MjSpec& self) -> py::list {
+      py::list list;
+      raw::MjsElement* el = mjs_firstElement(self.ptr, mjOBJ_ACTUATOR);
+      while (el) {
+        list.append(mjs_asActuator(el));
+        el = mjs_nextElement(self.ptr, el);
+      }
+      return list;
+    },
+    py::return_value_policy::reference_internal);
+  mjSpec.def_property_readonly(
+    "sensors",
+    [](MjSpec& self) -> py::list {
+      py::list list;
+      raw::MjsElement* el = mjs_firstElement(self.ptr, mjOBJ_SENSOR);
+      while (el) {
+        list.append(mjs_asSensor(el));
+        el = mjs_nextElement(self.ptr, el);
+      }
+      return list;
+    },
+    py::return_value_policy::reference_internal);
+  mjSpec.def_property_readonly(
+    "flexes",
+    [](MjSpec& self) -> py::list {
+      py::list list;
+      raw::MjsElement* el = mjs_firstElement(self.ptr, mjOBJ_FLEX);
+      while (el) {
+        list.append(mjs_asFlex(el));
+        el = mjs_nextElement(self.ptr, el);
+      }
+      return list;
+    },
+    py::return_value_policy::reference_internal);
+  mjSpec.def_property_readonly(
+    "pairs",
+    [](MjSpec& self) -> py::list {
+      py::list list;
+      raw::MjsElement* el = mjs_firstElement(self.ptr, mjOBJ_PAIR);
+      while (el) {
+        list.append(mjs_asPair(el));
+        el = mjs_nextElement(self.ptr, el);
+      }
+      return list;
+    },
+    py::return_value_policy::reference_internal);
+  mjSpec.def_property_readonly(
+    "equality",
+    [](MjSpec& self) -> py::list {
+      py::list list;
+      raw::MjsElement* el = mjs_firstElement(self.ptr, mjOBJ_EQUALITY);
+      while (el) {
+        list.append(mjs_asEquality(el));
+        el = mjs_nextElement(self.ptr, el);
+      }
+      return list;
+    },
+    py::return_value_policy::reference_internal);
+  mjSpec.def_property_readonly(
+    "excludes",
+    [](MjSpec& self) -> py::list {
+      py::list list;
+      raw::MjsElement* el = mjs_firstElement(self.ptr, mjOBJ_EXCLUDE);
+      while (el) {
+        list.append(mjs_asExclude(el));
+        el = mjs_nextElement(self.ptr, el);
+      }
+      return list;
+    },
+    py::return_value_policy::reference_internal);
+  mjSpec.def_property_readonly(
+    "tendons",
+    [](MjSpec& self) -> py::list {
+      py::list list;
+      raw::MjsElement* el = mjs_firstElement(self.ptr, mjOBJ_TENDON);
+      while (el) {
+        list.append(mjs_asTendon(el));
+        el = mjs_nextElement(self.ptr, el);
+      }
+      return list;
+    },
+    py::return_value_policy::reference_internal);
+  mjSpec.def_property_readonly(
+    "numeric",
+    [](MjSpec& self) -> py::list {
+      py::list list;
+      raw::MjsElement* el = mjs_firstElement(self.ptr, mjOBJ_NUMERIC);
+      while (el) {
+        list.append(mjs_asNumeric(el));
+        el = mjs_nextElement(self.ptr, el);
+      }
+      return list;
+    },
+    py::return_value_policy::reference_internal);
+  mjSpec.def_property_readonly(
+    "text",
+    [](MjSpec& self) -> py::list {
+      py::list list;
+      raw::MjsElement* el = mjs_firstElement(self.ptr, mjOBJ_TEXT);
+      while (el) {
+        list.append(mjs_asText(el));
+        el = mjs_nextElement(self.ptr, el);
+      }
+      return list;
+    },
+    py::return_value_policy::reference_internal);
+  mjSpec.def_property_readonly(
+    "tuple",
+    [](MjSpec& self) -> py::list {
+      py::list list;
+      raw::MjsElement* el = mjs_firstElement(self.ptr, mjOBJ_TUPLE);
+      while (el) {
+        list.append(mjs_asTuple(el));
+        el = mjs_nextElement(self.ptr, el);
+      }
+      return list;
+    },
+    py::return_value_policy::reference_internal);
+  mjSpec.def_property_readonly(
+    "key",
+    [](MjSpec& self) -> py::list {
+      py::list list;
+      raw::MjsElement* el = mjs_firstElement(self.ptr, mjOBJ_KEY);
+      while (el) {
+        list.append(mjs_asKey(el));
+        el = mjs_nextElement(self.ptr, el);
+      }
+      return list;
+    },
+    py::return_value_policy::reference_internal);
+  mjSpec.def_property_readonly(
+    "mesh",
+    [](MjSpec& self) -> py::list {
+      py::list list;
+      raw::MjsElement* el = mjs_firstElement(self.ptr, mjOBJ_MESH);
+      while (el) {
+        list.append(mjs_asMesh(el));
+        el = mjs_nextElement(self.ptr, el);
+      }
+      return list;
+    },
+    py::return_value_policy::reference_internal);
+  mjSpec.def_property_readonly(
+    "hfield",
+    [](MjSpec& self) -> py::list {
+      py::list list;
+      raw::MjsElement* el = mjs_firstElement(self.ptr, mjOBJ_HFIELD);
+      while (el) {
+        list.append(mjs_asHField(el));
+        el = mjs_nextElement(self.ptr, el);
+      }
+      return list;
+    },
+    py::return_value_policy::reference_internal);
+  mjSpec.def_property_readonly(
+    "skin",
+    [](MjSpec& self) -> py::list {
+      py::list list;
+      raw::MjsElement* el = mjs_firstElement(self.ptr, mjOBJ_SKIN);
+      while (el) {
+        list.append(mjs_asSkin(el));
+        el = mjs_nextElement(self.ptr, el);
+      }
+      return list;
+    },
+    py::return_value_policy::reference_internal);
+  mjSpec.def_property_readonly(
+    "texture",
+    [](MjSpec& self) -> py::list {
+      py::list list;
+      raw::MjsElement* el = mjs_firstElement(self.ptr, mjOBJ_TEXTURE);
+      while (el) {
+        list.append(mjs_asTexture(el));
+        el = mjs_nextElement(self.ptr, el);
+      }
+      return list;
+    },
+    py::return_value_policy::reference_internal);
+  mjSpec.def_property_readonly(
+    "material",
+    [](MjSpec& self) -> py::list {
+      py::list list;
+      raw::MjsElement* el = mjs_firstElement(self.ptr, mjOBJ_MATERIAL);
+      while (el) {
+        list.append(mjs_asMaterial(el));
+        el = mjs_nextElement(self.ptr, el);
+      }
+      return list;
+    },
+    py::return_value_policy::reference_internal);
 
   // ============================= MJSBODY ====================================
   mjsBody.def("id", [](raw::MjsBody& self) -> int {

python/mujoco/specs_test.py

@@ -217,6 +217,19 @@ class SpecsTest(absltest.TestCase):
         </mujoco>
     """))
 
+  def test_element_list(self):
+    spec = mujoco.MjSpec()
+    sensor1 = spec.add_sensor()
+    sensor2 = spec.add_sensor()
+    sensor3 = spec.add_sensor()
+    sensor1.name = 'sensor1'
+    sensor2.name = 'sensor2'
+    sensor3.name = 'sensor3'
+    self.assertLen(spec.sensors, 3)
+    self.assertEqual(spec.sensors[0].name, 'sensor1')
+    self.assertEqual(spec.sensors[1].name, 'sensor2')
+    self.assertEqual(spec.sensors[2].name, 'sensor3')
+
 
 if __name__ == '__main__':
   absltest.main()

src/user/user_api.cc

@@ -590,6 +590,22 @@ mjsElement* mjs_nextChild(mjsBody* body, mjsElement* child) {
 
 
 
+// return spec's first element of selected type
+mjsElement* mjs_firstElement(mjSpec* s, mjtObj type) {
+  mjCModel* modelC = static_cast<mjCModel*>(s->element);
+  return modelC->NextObject(NULL, type);
+}
+
+
+
+// return spec's next element; return NULL if element is last
+mjsElement* mjs_nextElement(mjSpec* s, mjsElement* element) {
+  mjCModel* modelC = static_cast<mjCModel*>(s->element);
+  return modelC->NextObject(element);
+}
+
+
+
 // return body given mjsElement
 mjsBody* mjs_asBody(mjsElement* element) {
   return element->elemtype == mjOBJ_BODY ? &(static_cast<mjCBody*>(element)->spec) : nullptr;
@@ -639,6 +655,118 @@ mjsFrame* mjs_asFrame(mjsElement* element) {
 
 
 
+// return actuator given mjsElement
+mjsActuator* mjs_asActuator(mjsElement* element) {
+  return element->elemtype == mjOBJ_ACTUATOR ? &(static_cast<mjCActuator*>(element)->spec) : nullptr;
+}
+
+
+
+// return sensor given mjsElement
+mjsSensor* mjs_asSensor(mjsElement* element) {
+  return element->elemtype == mjOBJ_SENSOR ? &(static_cast<mjCSensor*>(element)->spec) : nullptr;
+}
+
+
+
+// return flex given mjsElement
+mjsFlex* mjs_asFlex(mjsElement* element) {
+  return element->elemtype == mjOBJ_FLEX ? &(static_cast<mjCFlex*>(element)->spec) : nullptr;
+}
+
+
+
+// return pair given mjsElement
+mjsPair* mjs_asPair(mjsElement* element) {
+  return element->elemtype == mjOBJ_PAIR ? &(static_cast<mjCPair*>(element)->spec) : nullptr;
+}
+
+
+
+// return equality given mjsElement
+mjsEquality* mjs_asEquality(mjsElement* element) {
+  return element->elemtype == mjOBJ_EQUALITY ? &(static_cast<mjCEquality*>(element)->spec) : nullptr;
+}
+
+
+
+// return exclude given mjsElement
+mjsExclude* mjs_asExclude(mjsElement* element) {
+  return element->elemtype == mjOBJ_EXCLUDE ? &(static_cast<mjCBodyPair*>(element)->spec) : nullptr;
+}
+
+
+
+// return tendon given mjsElement
+mjsTendon* mjs_asTendon(mjsElement* element) {
+  return element->elemtype == mjOBJ_TENDON ? &(static_cast<mjCTendon*>(element)->spec) : nullptr;
+}
+
+
+
+// return numeric given mjsElement
+mjsNumeric* mjs_asNumeric(mjsElement* element) {
+  return element->elemtype == mjOBJ_NUMERIC ? &(static_cast<mjCNumeric*>(element)->spec) : nullptr;
+}
+
+
+
+// return text given mjsElement
+mjsText* mjs_asText(mjsElement* element) {
+  return element->elemtype == mjOBJ_TEXT ? &(static_cast<mjCText*>(element)->spec) : nullptr;
+}
+
+
+
+// return tuple given mjsElement
+mjsTuple* mjs_asTuple(mjsElement* element) {
+  return element->elemtype == mjOBJ_TUPLE ? &(static_cast<mjCTuple*>(element)->spec) : nullptr;
+}
+
+
+
+// return key given mjsElement
+mjsKey* mjs_asKey(mjsElement* element) {
+  return element->elemtype == mjOBJ_KEY ? &(static_cast<mjCKey*>(element)->spec) : nullptr;
+}
+
+
+
+// return mesh given mjsElement
+mjsMesh* mjs_asMesh(mjsElement* element) {
+  return element->elemtype == mjOBJ_MESH ? &(static_cast<mjCMesh*>(element)->spec) : nullptr;
+}
+
+
+
+// return hfield given mjsElement
+mjsHField* mjs_asHField(mjsElement* element) {
+  return element->elemtype == mjOBJ_HFIELD ? &(static_cast<mjCHField*>(element)->spec) : nullptr;
+}
+
+
+
+// return skin given mjsElement
+mjsSkin* mjs_asSkin(mjsElement* element) {
+  return element->elemtype == mjOBJ_SKIN ? &(static_cast<mjCSkin*>(element)->spec) : nullptr;
+}
+
+
+
+// return texture given mjsElement
+mjsTexture* mjs_asTexture(mjsElement* element) {
+  return element->elemtype == mjOBJ_TEXTURE ? &(static_cast<mjCTexture*>(element)->spec) : nullptr;
+}
+
+
+
+// return material given mjsElement
+mjsMaterial* mjs_asMaterial(mjsElement* element) {
+  return element->elemtype == mjOBJ_MATERIAL ? &(static_cast<mjCMaterial*>(element)->spec) : nullptr;
+}
+
+
+
 // set string
 void mjs_setString(mjString* dest, const char* text) {
   std::string* str = static_cast<std::string*>(dest);

src/user/user_api.h

@@ -218,6 +218,12 @@ MJAPI mjsElement* mjs_firstChild(mjsBody* body, mjtObj type);
 // Return body's next child of the same type; return NULL if child is last.
 MJAPI mjsElement* mjs_nextChild(mjsBody* body, mjsElement* child);
 
+// Return spec's first element of selected type.
+MJAPI mjsElement* mjs_firstElement(mjSpec* s, mjtObj type);
+
+// Return spec's next element; return NULL if element is last.
+MJAPI mjsElement* mjs_nextElement(mjSpec* s, mjsElement* element);
+
 // Safely cast an element as mjsBody, or return NULL if the element is not an mjsBody.
 MJAPI mjsBody* mjs_asBody(mjsElement* element);
 
@@ -239,6 +245,54 @@ MJAPI mjsLight* mjs_asLight(mjsElement* element);
 // Safely cast an element as mjsFrame, or return NULL if the element is not an mjsFrame.
 MJAPI mjsFrame* mjs_asFrame(mjsElement* element);
 
+// Safely cast an element as mjsActuator, or return NULL if the element is not an mjsActuator.
+MJAPI mjsActuator* mjs_asActuator(mjsElement* element);
+
+// Safely cast an element as mjsSensor, or return NULL if the element is not an mjsSensor.
+MJAPI mjsSensor* mjs_asSensor(mjsElement* element);
+
+// Safely cast an element as mjsFlex, or return NULL if the element is not an mjsFlex.
+MJAPI mjsFlex* mjs_asFlex(mjsElement* element);
+
+// Safely cast an element as mjsPair, or return NULL if the element is not an mjsPair.
+MJAPI mjsPair* mjs_asPair(mjsElement* element);
+
+// Safely cast an element as mjsEquality, or return NULL if the element is not an mjsEquality.
+MJAPI mjsEquality* mjs_asEquality(mjsElement* element);
+
+// Safely cast an element as mjsExclude, or return NULL if the element is not an mjsExclude.
+MJAPI mjsExclude* mjs_asExclude(mjsElement* element);
+
+// Safely cast an element as mjsTendon, or return NULL if the element is not an mjsTendon.
+MJAPI mjsTendon* mjs_asTendon(mjsElement* element);
+
+// Safely cast an element as mjsNumeric, or return NULL if the element is not an mjsNumeric.
+MJAPI mjsNumeric* mjs_asNumeric(mjsElement* element);
+
+// Safely cast an element as mjsText, or return NULL if the element is not an mjsText.
+MJAPI mjsText* mjs_asText(mjsElement* element);
+
+// Safely cast an element as mjsTuple, or return NULL if the element is not an mjsTuple.
+MJAPI mjsTuple* mjs_asTuple(mjsElement* element);
+
+// Safely cast an element as mjsKey, or return NULL if the element is not an mjsKey.
+MJAPI mjsKey* mjs_asKey(mjsElement* element);
+
+// Safely cast an element as mjsMesh, or return NULL if the element is not an mjsMesh.
+MJAPI mjsMesh* mjs_asMesh(mjsElement* element);
+
+// Safely cast an element as mjsHField, or return NULL if the element is not an mjsHField.
+MJAPI mjsHField* mjs_asHField(mjsElement* element);
+
+// Safely cast an element as mjsSkin, or return NULL if the element is not an mjsSkin.
+MJAPI mjsSkin* mjs_asSkin(mjsElement* element);
+
+// Safely cast an element as mjsTexture, or return NULL if the element is not an mjsTexture.
+MJAPI mjsTexture* mjs_asTexture(mjsElement* element);
+
+// Safely cast an element as mjsMaterial, or return NULL if the element is not an mjsMaterial.
+MJAPI mjsMaterial* mjs_asMaterial(mjsElement* element);
+
 
 //---------------------------------- Attribute setters ---------------------------------------------
 

src/user/user_model.cc

@@ -706,6 +706,70 @@ mjCBase* mjCModel::GetObject(mjtObj type, int id) {
 
 
 
+template <class T>
+static mjsElement* GetNext(std::vector<T*>& list, mjsElement* child) {
+  // TODO: use id for direct indexing instead of a loop
+  for (unsigned int i = 0; i < list.size()-1; i++) {
+    if (list[i]->spec.element == child) {
+      return list[i+1]->spec.element;
+    }
+  }
+  return nullptr;
+}
+
+
+
+// next object of specified type
+mjsElement* mjCModel::NextObject(mjsElement* object, mjtObj type) {
+  if (type == mjOBJ_UNKNOWN) {
+    if (!object) {
+      throw mjCError(NULL, "type must be specified if no element is given");
+    } else {
+      type = object->elemtype;
+    }
+  } else if (object && object->elemtype != type) {
+    throw mjCError(NULL, "element is not of requested type");
+  }
+
+  switch (type) {
+    case mjOBJ_ACTUATOR:
+      return object ? GetNext(actuators_, object) : actuators_[0];
+    case mjOBJ_SENSOR:
+      return object ? GetNext(sensors_, object) : sensors_[0];
+    case mjOBJ_FLEX:
+      return object ? GetNext(flexes_, object) : flexes_[0];
+    case mjOBJ_PAIR:
+      return object ? GetNext(pairs_, object) : pairs_[0];
+    case mjOBJ_EXCLUDE:
+      return object ? GetNext(excludes_, object) : excludes_[0];
+    case mjOBJ_EQUALITY:
+      return object ? GetNext(equalities_, object) : equalities_[0];
+    case mjOBJ_TENDON:
+      return object ? GetNext(tendons_, object) : tendons_[0];
+    case mjOBJ_NUMERIC:
+      return object ? GetNext(numerics_, object) : numerics_[0];
+    case mjOBJ_TEXT:
+      return object ? GetNext(texts_, object) : texts_[0];
+    case mjOBJ_TUPLE:
+      return object ? GetNext(tuples_, object) : tuples_[0];
+    case mjOBJ_KEY:
+      return object ? GetNext(keys_, object) : keys_[0];
+    case mjOBJ_MESH:
+      return object ? GetNext(meshes_, object) : meshes_[0];
+    case mjOBJ_HFIELD:
+      return object ? GetNext(hfields_, object) : hfields_[0];
+    case mjOBJ_SKIN:
+      return object ? GetNext(skins_, object) : skins_[0];
+    case mjOBJ_TEXTURE:
+      return object ? GetNext(textures_, object) : textures_[0];
+    case mjOBJ_MATERIAL:
+      return object ? GetNext(materials_, object) : materials_[0];
+    default:
+      return nullptr;
+  }
+}
+
+
 //------------------------ API FOR ACCESS TO PRIVATE VARIABLES -------------------------------------
 
 // compiled flag

src/user/user_model.h

@@ -203,6 +203,7 @@ class mjCModel : public mjCModel_, private mjSpec {
   // API for access to model elements (outside tree)
   int NumObjects(mjtObj type);              // number of objects in specified list
   mjCBase* GetObject(mjtObj type, int id);  // pointer to specified object
+  mjsElement* NextObject(mjsElement* object, mjtObj type = mjOBJ_UNKNOWN);  // next object of specified type
 
   // API for access to other variables
   bool IsCompiled() const;                                         // is model already compiled