Add Labels.skeleton convenience attribute (#54)

sleap_io/model/labels.py

@@ -190,6 +190,19 @@ def video(self) -> Video:
                 "in the labels. Use Labels.videos instead."
             )
 
+    @property
+    def skeleton(self) -> Skeleton:
+        """Return the skeleton if there is only a single skeleton in the labels."""
+        if len(self.skeletons) == 0:
+            raise ValueError("There are no skeletons in the labels.")
+        elif len(self.skeletons) == 1:
+            return self.skeletons[0]
+        else:
+            raise ValueError(
+                "Labels.skeleton can only be used when there is only a single skeleton "
+                "saved in the labels. Use Labels.skeletons instead."
+            )
+
     def find(
         self,
         video: Video,

sleap_io/model/skeleton.py

@@ -86,7 +86,7 @@ def _update_node_map(self, attr, nodes):
         self._node_name_map = {node.name: node for node in nodes}
         self._node_ind_map = {node: i for i, node in enumerate(nodes)}
 
-    nodes: list[Node] = field(on_setattr=_update_node_map)
+    nodes: list[Node] = field(factory=list, on_setattr=_update_node_map)
     edges: list[Edge] = field(factory=list)
     symmetries: list[Symmetry] = field(factory=list)
     name: Optional[str] = None

tests/model/test_labels.py

@@ -101,3 +101,18 @@ def test_labels_video():
     labels.videos.append(Video(filename="test2"))
     with pytest.raises(ValueError):
         labels.video
+
+
+def test_labels_skeleton():
+    labels = Labels()
+
+    with pytest.raises(ValueError):
+        labels.skeleton
+
+    skel = Skeleton(["A"])
+    labels.skeletons.append(skel)
+    assert labels.skeleton == skel
+
+    labels.skeletons.append(Skeleton(["B"]))
+    with pytest.raises(ValueError):
+        labels.skeleton