Initialize Leaf bracket depth

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_tokenize.py

@@ -6,6 +6,12 @@
 
 import black
 from blib2to3.pgen2 import token, tokenize
+from blib2to3.pytree import Leaf
+
+if sys.version_info >= (3, 11):
+    from typing import assert_type
+else:
+    from typing_extensions import assert_type
 
 
 @dataclass
@@ -30,6 +36,14 @@ def assert_tokenizes(text: str, tokens: list[Token]) -> None:
     assert actual_tokens == tokens
 
 
+def test_leaf_initializes_bracket_metadata() -> None:
+    leaf = Leaf(token.NAME, "name")
+
+    assert_type(leaf.bracket_depth, int)
+    assert leaf.bracket_depth == 0
+    assert leaf.opening_bracket is None
+
+
 def test_simple() -> None:
     assert_tokenizes(
         "1",