From 623e2f6690680a68db01f79a2210eb25cfa932ea Mon Sep 17 00:00:00 2001
From: Sai Asish Y <say.apm35@gmail.com>
Date: Tue, 12 May 2026 15:29:15 -0700
Subject: [PATCH] fix: report parser errors instead of crashing with IndexError

---
 CHANGELOG.md           | 2 ++
 yapf/yapflib/errors.py | 5 +++++
 yapftests/yapf_test.py | 6 ++++++
 3 files changed, 13 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index ccc8726c9..cbffb9886 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -75,6 +75,8 @@
   by taking precedence over SPLIT_BEFORE_NAMED_ASSIGNS.
 - Fix SPLIT_ALL_COMMA_SEPARATED_VALUES and SPLIT_ALL_TOP_LEVEL_COMMA_SEPARATED_VALUES
   being too agressive for lambdas and unpacking.
+- Report parser errors as a `YapfError` with a location instead of crashing with
+  an unrelated `IndexError`.
 
 ## [0.40.2] 2023-09-22
 ### Changes
diff --git a/yapf/yapflib/errors.py b/yapf/yapflib/errors.py
index 3a0102368..748c3bda7 100644
--- a/yapf/yapflib/errors.py
+++ b/yapf/yapflib/errors.py
@@ -13,6 +13,7 @@
 # limitations under the License.
 """YAPF error objects."""
 
+from yapf_third_party._ylib2to3.pgen2 import parse
 from yapf_third_party._ylib2to3.pgen2 import tokenize
 
 
@@ -34,6 +35,10 @@ def FormatErrorMsg(e):
   if isinstance(e, tokenize.TokenError):
     return '{}:{}:{}: {}'.format(e.filename, e.args[1][0], e.args[1][1],
                                  e.args[0])
+  if isinstance(e, parse.ParseError):
+    lineno, column = e.context[1]
+    return '{}:{}:{}: {}'.format(
+        getattr(e, 'filename', None), lineno, column, str(e))
   return '{}:{}:{}: {}'.format(e.args[1][0], e.args[1][1], e.args[1][2], e.msg)
 
 
diff --git a/yapftests/yapf_test.py b/yapftests/yapf_test.py
index a9ca01188..177e0460f 100644
--- a/yapftests/yapf_test.py
+++ b/yapftests/yapf_test.py
@@ -1565,6 +1565,12 @@ def testBadCode(self):
     code = 'x = """hello\n'
     self.assertRaises(errors.YapfError, yapf_api.FormatCode, code)
 
+  def testParseErrorReportedAsYapfError(self):
+    code = 'f"{tab["SOME_STRING"]}"\n'
+    with self.assertRaises(errors.YapfError) as ctx:
+      yapf_api.FormatCode(code)
+    self.assertRegex(str(ctx.exception), r':\d+:\d+: ')
+
 
 class DiffIndentTest(yapf_test_helper.YAPFTest):