From d81d625421aa160746c9dc005f8667f114473938 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?B=C3=A9n=C3=A9dikt=20Tran?=
 <10796600+picnixz@users.noreply.github.com>
Date: Tue, 14 Apr 2026 00:32:54 +0200
Subject: [PATCH] gh-148370: prevent quadratic behavior in
 `configparser.ParsingError.combine` (GH-148452) (cherry picked from commit
 2662db0c45aa16232136628457a53681b6683c25)
MIME-Version: 1.0
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Content-Transfer-Encoding: 8bit

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
---
 Lib/configparser.py                                 |  9 ++++++---
 Lib/test/test_configparser.py                       | 13 +++++++++++++
 .../2026-04-12-16-40-11.gh-issue-148370.0Li2EK.rst  |  2 ++
 3 files changed, 21 insertions(+), 3 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-04-12-16-40-11.gh-issue-148370.0Li2EK.rst

diff --git a/Lib/configparser.py b/Lib/configparser.py
index e76647d339e913..a53ac87276445a 100644
--- a/Lib/configparser.py
+++ b/Lib/configparser.py
@@ -315,12 +315,15 @@ def __init__(self, source, *args):
 
     def append(self, lineno, line):
         self.errors.append((lineno, line))
-        self.message += '\n\t[line %2d]: %s' % (lineno, repr(line))
+        self.message += f'\n\t[line {lineno:2d}]: {line!r}'
 
     def combine(self, others):
+        messages = [self.message]
         for other in others:
-            for error in other.errors:
-                self.append(*error)
+            for lineno, line in other.errors:
+                self.errors.append((lineno, line))
+                messages.append(f'\n\t[line {lineno:2d}]: {line!r}')
+        self.message = "".join(messages)
         return self
 
     @staticmethod
diff --git a/Lib/test/test_configparser.py b/Lib/test/test_configparser.py
index d7c4f19c1a5ef0..8d8dd2a2bf27fb 100644
--- a/Lib/test/test_configparser.py
+++ b/Lib/test/test_configparser.py
@@ -1729,6 +1729,19 @@ def test_error(self):
             self.assertEqual(e1.message, e2.message)
             self.assertEqual(repr(e1), repr(e2))
 
+    def test_combine_error_linear_complexity(self):
+        # Ensure that ParsingError.combine() has linear complexity.
+        # See https://github.com/python/cpython/issues/148370.
+        n = 50000
+        s = '[*]\n' + (err_line := '=\n') * n
+        p = configparser.ConfigParser(strict=False)
+        with self.assertRaises(configparser.ParsingError) as cm:
+            p.read_string(s)
+        errlines = cm.exception.message.splitlines()
+        self.assertEqual(len(errlines), n + 1)
+        self.assertStartsWith(errlines[0], "Source contains parsing errors: ")
+        self.assertEqual(errlines[42], f"\t[line {43:2d}]: {err_line!r}")
+
     def test_nosectionerror(self):
         import pickle
         e1 = configparser.NoSectionError('section')
diff --git a/Misc/NEWS.d/next/Library/2026-04-12-16-40-11.gh-issue-148370.0Li2EK.rst b/Misc/NEWS.d/next/Library/2026-04-12-16-40-11.gh-issue-148370.0Li2EK.rst
new file mode 100644
index 00000000000000..3bb662350796f6
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-04-12-16-40-11.gh-issue-148370.0Li2EK.rst
@@ -0,0 +1,2 @@
+:mod:`configparser`: prevent quadratic behavior when a :exc:`~configparser.ParsingError`
+is raised after a parser fails to parse multiple lines. Patch by Bénédikt Tran.