From 29c3137b5533481c5c36832b856622206f0a13cf Mon Sep 17 00:00:00 2001
From: Sergey B Kirpichev <skirpichev@gmail.com>
Date: Tue, 15 Sep 2026 16:45:12 +0300
Subject: [PATCH] [3.15] gh-155526: Don't check errno in abs(complex)
 (GH-155527) (#157341)

Co-authored-by: Victor Stinner <vstinner@python.org>
Co-authored-by: hpkfft.com <paul@hpkfft.com>
(cherry picked from commit 340cfbaf679c1230fd438042d764b6922b483242)
---
 Lib/test/test_complex.py                      | 25 +++++++++++++++++++
 ...-08-11-06-04-26.gh-issue-155526.W7ZHXu.rst |  3 +++
 Modules/cmathmodule.c                         |  2 +-
 Objects/complexobject.c                       |  1 +
 4 files changed, 30 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Core_and_Builtins/2026-08-11-06-04-26.gh-issue-155526.W7ZHXu.rst

diff --git a/Lib/test/test_complex.py b/Lib/test/test_complex.py
index af4eba5451ba196..8ba120727657247 100644
--- a/Lib/test/test_complex.py
+++ b/Lib/test/test_complex.py
@@ -1,6 +1,8 @@
+import errno
 import unittest
 import sys
 from test import support
+from test.support import import_helper
 from test.support.testcase import ComplexesAreIdenticalMixin
 from test.support.numbers import (
     VALID_UNDERSCORE_LITERALS,
@@ -9,6 +11,7 @@
 
 from random import random
 from math import isnan, copysign
+import cmath
 import operator
 
 INF = float("inf")
@@ -613,8 +616,30 @@ def test_abs(self):
         for num in nums:
             self.assertAlmostEqual((num.real**2 + num.imag**2)  ** 0.5, abs(num))
 
+        for x in 0.0, -0.0, INF, -INF, NAN:
+            for y in 0.0, -0.0, INF, -INF, NAN:
+                with self.subTest(x=x, y=y):
+                    z = complex(x, y)
+                    r = abs(z)
+                    if cmath.isfinite(z):
+                        self.assertFloatsAreIdentical(r, 0.0)
+                    elif cmath.isinf(z):
+                        self.assertEqual(r, INF)
+                    else:
+                        self.assertTrue(cmath.isnan(z))
+                        self.assertTrue(isnan(r))
+
         self.assertRaises(OverflowError, abs, complex(DBL_MAX, DBL_MAX))
 
+    def test_abs_errno_handling(self):
+        _testcapi = import_helper.import_module('_testcapi')
+        z = complex('nan')
+        _testcapi.set_errno(errno.ERANGE)
+        try:
+            self.assertTrue(isnan(abs(z)))
+        finally:
+            _testcapi.set_errno(0)
+
     def test_repr_str(self):
         def test(v, expected, test_fn=self.assertEqual):
             test_fn(repr(v), expected)
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-08-11-06-04-26.gh-issue-155526.W7ZHXu.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-08-11-06-04-26.gh-issue-155526.W7ZHXu.rst
new file mode 100644
index 000000000000000..ae0c08973a8195b
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2026-08-11-06-04-26.gh-issue-155526.W7ZHXu.rst
@@ -0,0 +1,3 @@
+Fix spurious :exc:`OverflowError` for ``abs(nanj)`` in case :c:data:`errno` was
+previously set to :c:macro:`!ERANGE` by some library call.
+Patch by Sergey B Kirpichev.
diff --git a/Modules/cmathmodule.c b/Modules/cmathmodule.c
index affb717f31b1100..538dc1c0e22425b 100644
--- a/Modules/cmathmodule.c
+++ b/Modules/cmathmodule.c
@@ -991,8 +991,8 @@ cmath_polar_impl(PyObject *module, Py_complex z)
 {
     double r, phi;
 
-    errno = 0;
     phi = c_atan2(z); /* should not cause any exception */
+    errno = 0;
     r = _Py_c_abs(z); /* sets errno to ERANGE on overflow */
     if (errno != 0)
         return math_error();
diff --git a/Objects/complexobject.c b/Objects/complexobject.c
index 502b4a9a96119e7..a0685f0ee7de21c 100644
--- a/Objects/complexobject.c
+++ b/Objects/complexobject.c
@@ -589,6 +589,7 @@ complex_abs(PyComplexObject *v)
 {
     double result;
 
+    errno = 0;
     result = _Py_c_abs(v->cval);
 
     if (errno == ERANGE) {