Add worldbody assertions in attach tests.

This clarifies the difference between attaching a full spec, which converts the worldbody to a frame, and attaching a specific body (or frame).

PiperOrigin-RevId: 721297010
Change-Id: Iff87fe99b81746e6ab3a189a3057b274d3c9a939

python/mujoco/specs_test.py

@@ -966,6 +966,7 @@ def test_attach_to_site(self):
     child1.assets = {'cube1.obj': 'cube1_content'}
     body1 = child1.worldbody.add_body()
     self.assertIs(body1, site.attach_body(body1, prefix='_'))
+    self.assertIsNotNone(child1.worldbody)
     body1.pos = [1, 1, 1]
     model1 = parent.compile()
     self.assertIsNotNone(model1)
@@ -979,6 +980,7 @@ def test_attach_to_site(self):
     child2.assets = {'cube2.obj': 'cube2_content'}
     body2 = child2.worldbody.add_body(name='body')
     self.assertIsNotNone(parent.attach(child2, site=site, prefix='child2-'))
+    self.assertIsNone(child2.worldbody)
     body2.pos = [-1, -1, -1]
     model2 = parent.compile()
     self.assertIsNotNone(model2)
@@ -995,6 +997,7 @@ def test_attach_to_site(self):
     child3.assets = {'cube3.obj': 'cube3_content'}
     body3 = child3.worldbody.add_body(name='body')
     self.assertIsNotNone(parent.attach(child3, site='site', prefix='child3-'))
+    self.assertIsNone(child3.worldbody)
     body3.pos = [-2, -2, -2]
     model3 = parent.compile()
     self.assertIsNotNone(model3)
@@ -1032,6 +1035,7 @@ def test_attach_to_frame(self):
     child1.assets = {'cube1.obj': 'cube1_content'}
     body1 = child1.worldbody.add_body()
     self.assertIs(body1, frame.attach_body(body1, prefix='_'))
+    self.assertIsNotNone(child1.worldbody)
     body1.pos = [1, 1, 1]
     model1 = parent.compile()
     self.assertIsNotNone(model1)
@@ -1045,6 +1049,7 @@ def test_attach_to_frame(self):
     child2.assets = {'cube2.obj': 'cube2_content'}
     body2 = child2.worldbody.add_body(name='body')
     self.assertIsNotNone(parent.attach(child2, frame=frame, prefix='child-'))
+    self.assertIsNone(child2.worldbody)
     body2.pos = [-1, -1, -1]
     model2 = parent.compile()
     self.assertIsNotNone(model2)
@@ -1061,6 +1066,7 @@ def test_attach_to_frame(self):
     child3.assets = {'cube2.obj': 'new_cube2_content'}
     body3 = child3.worldbody.add_body(name='body')
     self.assertIsNotNone(parent.attach(child3, frame='frame', prefix='child3-'))
+    self.assertIsNone(child3.worldbody)
     body3.pos = [-2, -2, -2]
     model3 = parent.compile()
     self.assertIsNotNone(model3)