Enable to attach a spec to a frame.

PiperOrigin-RevId: 699703988
Change-Id: Idbddcbce1e68286248458eb0bd24d5080451a58a

python/mujoco/specs_test.py

@@ -884,6 +884,19 @@ class SpecsTest(absltest.TestCase):
     frame = body.to_frame()
     np.testing.assert_array_equal(frame.pos, [1, 2, 3])
 
+  def test_attach_spec_to_frame(self):
+    child = mujoco.MjSpec()
+    child.worldbody.add_camera(name='camera')
+    parent = mujoco.MjSpec()
+    frame = parent.worldbody.add_frame(name='frame')
+    frame.attach(child, 'child-', '')
+    self.assertLen(child.cameras, 1)
+    self.assertLen(parent.bodies, 1)
+    self.assertLen(parent.frames, 2)
+    self.assertEqual(parent.cameras[0].name, 'child-camera')
+    self.assertEqual(parent.frames[0].name, 'frame')
+    self.assertEqual(parent.frames[1].name, '')
+
 
 if __name__ == '__main__':
   absltest.main()