Capture mjs_attach error message in the Python bindings.

PiperOrigin-RevId: 682409212
Change-Id: I391ee48626e7dcd985f65a231370ef5ec16124d7

python/mujoco/specs.cc

@@ -563,7 +563,12 @@ PYBIND11_MODULE(_specs, m) {
       "attach_frame",
       [](raw::MjsBody& self, raw::MjsFrame& frame, std::string& prefix,
          std::string& suffix) -> raw::MjsFrame* {
-        return mjs_attachFrame(&self, &frame, prefix.c_str(), suffix.c_str());
+        auto new_frame =
+            mjs_attachFrame(&self, &frame, prefix.c_str(), suffix.c_str());
+        if (!new_frame) {
+          throw pybind11::value_error(mjs_getError(mjs_getSpec(&self)));
+        }
+        return new_frame;
       },
       py::return_value_policy::reference_internal);
 
@@ -578,7 +583,13 @@ PYBIND11_MODULE(_specs, m) {
       "attach_body",
       [](raw::MjsFrame& self, raw::MjsBody& body, std::string& prefix,
          std::string& suffix) -> raw::MjsBody* {
-        return mjs_attachBody(&self, &body, prefix.c_str(), suffix.c_str());
+        auto new_body =
+            mjs_attachBody(&self, &body, prefix.c_str(), suffix.c_str());
+        if (!new_body) {
+          throw pybind11::value_error(
+              mjs_getError(mjs_getSpecFromFrame(&self)));
+        }
+        return new_body;
       },
       py::return_value_policy::reference_internal);
 

python/mujoco/specs_test.py

@@ -846,5 +846,20 @@ class SpecsTest(absltest.TestCase):
     with self.assertRaises(IndexError):
       material.textures[-1] = 'x'
 
+  def test_attach_error(self):
+    child = mujoco.MjSpec()
+    parent = mujoco.MjSpec()
+    parent.degree = not child.degree
+    body = parent.worldbody.add_body()
+    frame = child.worldbody.add_frame()
+    with self.assertRaises(ValueError) as cm:
+      body.attach_frame(frame, '', '')
+    self.assertEqual(
+        str(cm.exception),
+        'Error: cannot attach mjSpecs with incompatible compiler/angle'
+        ' attribute',
+    )
+
+
 if __name__ == '__main__':
   absltest.main()