Expose MjSpec raw pointer via _address

python/mujoco/specs.cc

@@ -393,6 +393,9 @@ PYBIND11_MODULE(_specs, m) {
   mjSpec.def("copy", [](const MjSpec& self) -> MjSpec {
     return MjSpec(self);
   });
+  mjSpec.def_property_readonly("_address", [](const MjSpec& self) {
+    return reinterpret_cast<uintptr_t>(self.ptr);
+  });
   mjSpec.def_property(
       "copy_during_attach",
       [](MjSpec& self) {

python/mujoco/specs_test.py

@@ -1159,5 +1159,15 @@ class SpecsTest(absltest.TestCase):
     ):
       model_spec.compile()
 
+  def test_address(self):
+    spec1 = mujoco.MjSpec()
+    spec2 = mujoco.MjSpec()
+    spec3 = spec1.copy()
+
+    self.assertGreater(spec1._address, 0)
+    self.assertGreater(spec2._address, 0)
+    self.assertGreater(spec3._address, 0)
+    self.assertLen({spec1._address, spec2._address, spec3._address}, 3)
+
 if __name__ == '__main__':
   absltest.main()