Added child to Tree

Author: mapio

src/liblet/display.py

@@ -303,6 +303,21 @@ def _to_tree(ast_node):
 
     return _to_tree(node)
 
+  @property
+  def child(self):
+    """Returns the only child of the tree, or raises an exception.
+
+    Returns:
+      The first child of the tree.
+    Raises:
+      ValueError: if the tree has no children or more than one child.
+    """
+    if len(self.children) == 1:
+      return self.children[0]
+    if len(self.children) == 0:
+      raise ValueError('No children available')
+    raise ValueError('More than one child present')
+
   def to_lol(self):
     def walk(T):
       return (T.root, *tuple(walk(child) for child in T.children))

src/tests/display_test.py

@@ -4,6 +4,18 @@
 
 
 class TestTree(unittest.TestCase):
+  def test_child(self):
+    t = Tree('root', ['child'])
+    self.assertEqual(t.child, 'child')
+
+  def test_two_children_raises_value_error(self):
+    with self.assertRaises(ValueError):
+      _ = Tree('root', ['child1', 'child2']).child
+
+  def test_no_children_raises_value_error(self):
+    with self.assertRaises(ValueError):
+      _ = Tree('root').child
+
   def test_from_lol(self):
     lol = (1, (11,), (12, (121,), (122,)))
     t = Tree.from_lol(lol)