pythongh-114678: Fix incorrect deprecation warning for 'N' specifier …

…in Decimal format

Co-authored-by: Stefan Krah <skrah@bytereef.org>

Lib/test/test_decimal.py

@@ -1237,7 +1237,10 @@ def test_deprecated_N_format(self):
         else:
             self.assertRaises(ValueError, format, h, 'N')
             self.assertRaises(ValueError, format, h, '010.3N')
-
+        self.assertEqual(format(h, 'N>10.3'), 'NN6.63E-34')
+        self.assertEqual(format(h, 'N>10.3n'), 'NN6.63e-34')
+        self.assertEqual(format(h, 'N>10.3e'), 'N6.626e-34')
+        self.assertEqual(format(h, 'N>10.3f'), 'NNNNN0.000')
 
     @run_with_locale('LC_ALL', 'ps_AF')
     def test_wide_char_separator_decimal_point(self):

Misc/NEWS.d/next/Library/2024-01-28-19-40-40.gh-issue-114678.kYKcJw.rst

@@ -0,0 +1,2 @@
+Fix incorrect deprecation warning for 'N' specifier in
+:class:`~decimal.Decimal` format. Based on patch by Stefan Krah.

Modules/_decimal/_decimal.c

@@ -3446,6 +3446,14 @@ dec_format(PyObject *dec, PyObject *args)
         if (fmt == NULL) {
             return NULL;
         }
+
+        if (size > 0 && fmt[size-1] == 'N') {
+            if (PyErr_WarnEx(PyExc_DeprecationWarning,
+                             "Format specifier 'N' is deprecated", 1) < 0) {
+                return NULL;
+            }
+        }
+
         /* NOTE: If https://github.com/python/cpython/pull/29438 lands, the
          *   format string manipulation below can be eliminated by enhancing
          *   the forked mpd_parse_fmt_str(). */
@@ -3593,12 +3601,6 @@ dec_format(PyObject *dec, PyObject *args)
     if (replace_fillchar) {
         dec_replace_fillchar(decstring);
     }
-    if (strchr(fmt, 'N') != NULL) {
-        if (PyErr_WarnEx(PyExc_DeprecationWarning,
-                         "Format specifier 'N' is deprecated", 1) < 0) {
-            goto finish;
-        }
-    }
 
     result = PyUnicode_DecodeUTF8(decstring, size, NULL);