Add read-only access to the children of a TreeNode

See #1398.

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [0.10.0] - Unreleased
 
+### Added
+
+- Added read-only public access to the children of a `TreeNode` via `TreeNode.children` https://github.com/Textualize/textual/issues/1398
+
 ### Changed
 
 - `MouseScrollUp` and `MouseScrollDown` now inherit from `MouseEvent` and have attached modifier keys. https://github.com/Textualize/textual/pull/1458
@@ -29,8 +33,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - Widget.render_line now returns a Strip
 - Fix for slow updates on Windows
-- Bumped Rich dependency 
+- Bumped Rich dependency
-  
+
 ## [0.8.2] - 2022-12-28
 
 ### Fixed

src/textual/widgets/_tree.py

@@ -14,6 +14,7 @@ from .._loop import loop_last
 from .._segment_tools import line_crop, line_pad
 from .._types import MessageTarget
 from .._typing import TypeAlias
+from .._collections import ImmutableSequence
 from ..binding import Binding
 from ..geometry import Region, Size, clamp
 from ..message import Message
@@ -53,6 +54,10 @@ class _TreeLine:
         return guides
 
 
+class TreeNodes(ImmutableSequence["TreeNode[TreeDataType]"]):
+    """An immutable collection of `TreeNode`."""
+
+
 @rich.repr.auto
 class TreeNode(Generic[TreeDataType]):
     """An object that represents a "node" in a tree control."""
@@ -74,7 +79,7 @@ class TreeNode(Generic[TreeDataType]):
         self._label = label
         self.data = data
         self._expanded = expanded
-        self._children: list[TreeNode] = []
+        self._children: list[TreeNode[TreeDataType]] = []
 
         self._hover_ = False
         self._selected_ = False
@@ -91,6 +96,11 @@ class TreeNode(Generic[TreeDataType]):
         self._selected_ = False
         self._updates += 1
 
+    @property
+    def children(self) -> TreeNodes[TreeDataType]:
+        """TreeNodes[TreeDataType]: The child nodes of a TreeNode."""
+        return TreeNodes(self._children)
+
     @property
     def line(self) -> int:
         """int: Get the line number for this node, or -1 if it is not displayed."""

tests/tree/test_tree_node_children.py

@@ -0,0 +1,32 @@
+import pytest
+from textual.widgets import Tree, TreeNode
+
+def label_of(node: TreeNode[None]):
+    """Get the label of a node.
+
+    TODO: This is just a helper function to reduce the number of type
+    errors, which can and will be remove once this code is merged with a
+    version of main that also has the TreeNode.label PR merged.
+    """
+    return str(node._label)
+
+
+def test_tree_node_children() -> None:
+    """A node's children property should act like an immutable list."""
+    CHILDREN=23
+    tree = Tree[None]("Root")
+    for child in range(CHILDREN):
+        tree.root.add(str(child))
+    assert len(tree.root.children)==CHILDREN
+    for child in range(CHILDREN):
+        assert label_of(tree.root.children[child]) == str(child)
+    assert label_of(tree.root.children[0]) == "0"
+    assert label_of(tree.root.children[-1]) == str(CHILDREN-1)
+    assert [label_of(node) for node in tree.root.children] == [str(n) for n in range(CHILDREN)]
+    assert [label_of(node) for node in tree.root.children[:2]] == [str(n) for n in range(2)]
+    with pytest.raises(TypeError):
+        tree.root.children[0] = tree.root.children[1]
+    with pytest.raises(TypeError):
+        del tree.root.children[0]
+    with pytest.raises(TypeError):
+        del tree.root.children[0:2]