Initialize Leaf bracket depth

Author: michaelsam94

CHANGES.md

@@ -17,6 +17,8 @@
 
 <!-- Changes that affect Black's stable style -->
 
+- Fix crash when a leaf's bracket metadata is read before bracket tracking has visited
+  it (#5215)
 - Fix crash when a standalone comment sits between tokens of a comprehension or lambda
   (#5144)
 - Fix crash when a comment-only `# fmt: off`/`# fmt: on` block is followed by a `with`

src/blib2to3/pytree.py

@@ -482,6 +482,7 @@ def __init__(
             self._prefix = prefix
         self.fixers_applied: list[Any] | None = fixers_applied[:]
         self.children = []
+        self.bracket_depth = 0
         self.opening_bracket = opening_bracket
         self.fmt_pass_converted_first_leaf = fmt_pass_converted_first_leaf
 

tests/test_pytree.py

@@ -0,0 +1,11 @@
+"""Tests for blib2to3 concrete syntax trees."""
+
+from blib2to3.pgen2 import token
+from blib2to3.pytree import Leaf
+
+
+def test_leaf_initializes_bracket_metadata() -> None:
+    leaf = Leaf(token.NAME, "name")
+
+    assert leaf.bracket_depth == 0
+    assert leaf.opening_bracket is None