gh-155526: correct errno handling in complex_abs()

Author: skirpichev

Lib/test/test_complex.py

@@ -1,6 +1,7 @@
 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,
@@ -791,6 +792,12 @@ def test_abs(self):
 
         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(34)
+        self.assertTrue(isnan(abs(z)))
+
     def test_repr_str(self):
         def test(v, expected, test_fn=self.assertEqual):
             test_fn(repr(v), expected)

Misc/NEWS.d/next/Core_and_Builtins/2026-08-11-06-04-26.gh-issue-155526.W7ZHXu.rst

@@ -0,0 +1,2 @@
+Correct ``errno`` handling in ``abs(complex)``.  Patch by Sergey B
+Kirpichev.

Objects/complexobject.c

@@ -367,7 +367,7 @@ c_powi(Py_complex x, long n)
 double
 _Py_c_abs(Py_complex z)
 {
-    /* sets errno = ERANGE on overflow;  otherwise errno = 0 */
+    /* sets errno = ERANGE on overflow */
     double result;
 
     if (!isfinite(z.real) || !isfinite(z.imag)) {
@@ -376,23 +376,20 @@ _Py_c_abs(Py_complex z)
            NaN. */
         if (isinf(z.real)) {
             result = fabs(z.real);
-            errno = 0;
             return result;
         }
         if (isinf(z.imag)) {
             result = fabs(z.imag);
-            errno = 0;
             return result;
         }
         /* either the real or imaginary part is a NaN,
            and neither is infinite. Result should be NaN. */
         return Py_NAN;
     }
     result = hypot(z.real, z.imag);
-    if (!isfinite(result))
+    if (!isfinite(result)) {
         errno = ERANGE;
-    else
-        errno = 0;
+    }
     return result;
 }
 
@@ -796,7 +793,10 @@ static PyObject *
 complex_abs(PyObject *op)
 {
     PyComplexObject *v = _PyComplexObject_CAST(op);
-    double result = _Py_c_abs(v->cval);
+    double result;
+
+    errno = 0;
+    result = _Py_c_abs(v->cval);
     if (errno == ERANGE) {
         PyErr_SetString(PyExc_OverflowError,
                         "absolute value too large");