gh-78724: deprecate incomplete initialization of struct.Struct()

* ``Struct.__new__()`` will require a mandatory argument (format)
* Calls of ``__init__()`` method on initialized Struct are deprecated

Author: skirpichev

Doc/deprecations/pending-removal-in-3.20.rst

@@ -1,6 +1,13 @@
 Pending removal in Python 3.20
 ------------------------------
 
+* Calling the ``Struct.__new__()`` without required argument now is
+  deprecated and will be removed in Python 3.20.  Calling
+  :meth:`~object.__init__` method on initialized :class:`~struct.Struct`
+  objects is deprecated and will be removed in Python 3.20.
+
+  (Contributed by Sergey B Kirpichev in :gh:`78724`.)
+
 * The ``__version__``, ``version`` and ``VERSION`` attributes have been
   deprecated in these standard library modules and will be removed in
   Python 3.20. Use :py:data:`sys.version_info` instead.

Doc/whatsnew/3.15.rst

@@ -1137,6 +1137,15 @@ New deprecations
 
     (Contributed by Bénédikt Tran in :gh:`134978`.)
 
+* :mod:`struct`:
+
+  * Calling the ``Struct.__new__()`` without required argument now is
+    deprecated and will be removed in Python 3.20.  Calling
+    :meth:`~object.__init__` method on initialized :class:`~struct.Struct`
+    objects is deprecated and will be removed in Python 3.20.
+
+    (Contributed by Sergey B Kirpichev in :gh:`78724`.)
+
 * ``__version__``
 
   * The ``__version__``, ``version`` and ``VERSION`` attributes have been

Lib/test/test_struct.py

@@ -7,6 +7,7 @@
 import unittest
 import struct
 import sys
+import warnings
 import weakref
 
 from test import support
@@ -575,7 +576,11 @@ def test_Struct_reinitialization(self):
         # Struct instance.  This test can be used to detect the leak
         # when running with regrtest -L.
         s = struct.Struct('i')
-        s.__init__('ii')
+        with self.assertWarns(DeprecationWarning):
+            s.__init__('ii')
+        with warnings.catch_warnings():
+            warnings.simplefilter("error", DeprecationWarning)
+            self.assertRaises(DeprecationWarning, s.__init__, 'ii')
 
     def check_sizeof(self, format_str, number_of_codes):
         # The size of 'PyStructObject'
@@ -783,7 +788,8 @@ class MyStruct(struct.Struct):
             def __init__(self):
                 super().__init__('>h')
 
-        my_struct = MyStruct()
+        with self.assertWarns(DeprecationWarning):
+            my_struct = MyStruct()
         self.assertEqual(my_struct.pack(12345), b'\x30\x39')
 
     def test_repr(self):

Misc/NEWS.d/next/Library/2026-01-10-16-23-21.gh-issue-78724.HZrfSA.rst

@@ -0,0 +1,3 @@
+Calling the ``Struct.__new__()`` without required argument now is deprecated.
+Calling :meth:`~object.__init__` method on initialized :class:`~struct.Struct`
+objects is deprecated.  Patch by Sergey B Kirpichev.

Modules/_struct.c

@@ -1762,6 +1762,12 @@ s_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 {
     PyObject *self;
 
+    if (PyTuple_GET_SIZE(args) != 1
+        && PyErr_WarnEx(PyExc_DeprecationWarning,
+                        "Struct().__new__() has one required argument", 1))
+    {
+        return NULL;
+    }
     assert(type != NULL);
     allocfunc alloc_func = PyType_GetSlot(type, Py_tp_alloc);
     assert(alloc_func != NULL);
@@ -1796,6 +1802,13 @@ Struct___init___impl(PyStructObject *self, PyObject *format)
 {
     int ret = 0;
 
+    if (self->s_codes
+        && PyErr_WarnEx(PyExc_DeprecationWarning,
+                        ("Explicit call of __init__() on "
+                         "initialized Struct() is deprecated"), 1))
+    {
+        return -1;
+    }
     if (PyUnicode_Check(format)) {
         format = PyUnicode_AsASCIIString(format);
         if (format == NULL)