gh-121249: Deprecate using F/D type codes in the struct module

Author: skirpichev

Doc/deprecations/pending-removal-in-3.21.rst

@@ -17,3 +17,9 @@ Pending removal in Python 3.21
     are not generated by the parser or accepted by the code generator.
   * The ``dims`` property of ``ast.Tuple`` will be removed in Python 3.21. Use
     the ``ast.Tuple.elts`` property instead.
+
+* :mod:`struct`:
+
+  * Soft-deprecated since Python 3.15 using ``'F'`` and ``'D'`` type codes now
+    deprecated.  These codes will be removed in Python 3.21.  Use instead
+    two-letter forms ``'Zf'`` and ``'Zd'``.

Doc/whatsnew/3.16.rst

@@ -485,6 +485,13 @@ New deprecations
     3.9, now issues a deprecation warning on use. This property is slated for
     removal in 3.21. Use ``ast.Tuple.elts`` instead.
 
+* :mod:`struct`:
+
+  * Soft-deprecated since Python 3.15 using ``'F'`` and ``'D'`` type codes now
+    deprecated.  These codes will be removed in Python 3.21.  Use instead
+    two-letter forms ``'Zf'`` and ``'Zd'``.
+    (Contributed by Sergey B Kirpichev in :gh:`121249`.)
+
 .. Add deprecations above alphabetically, not here at the end.
 
 .. include:: ../deprecations/pending-removal-in-3.17.rst

Lib/test/test_struct.py

@@ -7,6 +7,7 @@
 import unittest
 import struct
 import sys
+import warnings
 import weakref
 
 from test import support
@@ -995,14 +996,25 @@ def test_c_complex_round_trip(self):
         values = [complex(*_) for _ in combinations([1, -1, 0.0, -0.0, 2,
                                                      -3, INF, -INF, NAN], 2)]
         for z in values:
-            for f in [
-                'F', 'D', 'Zf', 'Zd',
-                '>F', '>D', '>Zf', '>Zd',
-                '<F', '<D', '<Zf', '<Zd',
-            ]:
+            for f in ['Zf', 'Zd', '>Zf', '>Zd', '<Zf', '<Zd']:
                 with self.subTest(z=z, format=f):
                     round_trip = struct.unpack(f, struct.pack(f, z))[0]
                     self.assertComplexesAreIdentical(z, round_trip)
+        for f in ['F', 'D', '>F', '>D', '<F', '<D']:
+            z = 1+1j
+            with self.subTest(format=f):
+                with warnings.catch_warnings():
+                    warnings.simplefilter("error", DeprecationWarning)
+                    self.assertRaises(DeprecationWarning, struct.pack, f, z)
+                with warnings.catch_warnings():
+                    with self.assertWarns(DeprecationWarning):
+                        b = struct.pack(f, z)
+                with warnings.catch_warnings():
+                    warnings.simplefilter("error", DeprecationWarning)
+                    self.assertRaises(DeprecationWarning, struct.unpack, f, b)
+                with self.assertWarns(DeprecationWarning):
+                    round_trip = struct.unpack(f, b)[0]
+                self.assertComplexesAreIdentical(z, round_trip)
 
     @unittest.skipIf(
         support.is_android or support.is_apple_mobile,

Misc/NEWS.d/next/Library/2026-06-26-19-01-13.gh-issue-121249.p1SBW0.rst

@@ -0,0 +1 @@
+Deprecate using ``'F'`` and ``'D'`` type codes in the :mod:`struct`.

Modules/_struct.c

@@ -1697,6 +1697,21 @@ prepare_s(PyStructObject *self, PyObject *format)
         if (e == NULL)
             return -1;
 
+        if (strcmp(e->format, "F") == 0) {
+            if (PyErr_WarnEx(PyExc_DeprecationWarning,
+                            "The 'F' type code is deprecated, use 'Zf'", 1))
+            {
+                return -1;
+            }
+        }
+        if (strcmp(e->format, "D") == 0) {
+            if (PyErr_WarnEx(PyExc_DeprecationWarning,
+                            "The 'D' type code is deprecated, use 'Zd'", 1))
+            {
+                return -1;
+            }
+        }
+
         switch (c) {
             case 's': _Py_FALLTHROUGH;
             case 'p':
@@ -2065,6 +2080,20 @@ s_unpack_internal(PyStructObject *soself, const char *startfrom,
                 }
                 v = PyBytes_FromStringAndSize(res + 1, n);
             } else {
+                if (strcmp(e->format, "F") == 0) {
+                    if (PyErr_WarnEx(PyExc_DeprecationWarning,
+                            "The 'F' type code is deprecated, use 'Zf'", 1))
+                    {
+                        goto fail;
+                    }
+                }
+                if (strcmp(e->format, "D") == 0) {
+                    if (PyErr_WarnEx(PyExc_DeprecationWarning,
+                            "The 'D' type code is deprecated, use 'Zd'", 1))
+                    {
+                        goto fail;
+                    }
+                }
                 v = e->unpack(state, res, e);
             }
             if (v == NULL)