gh-143715: deprecate incomplete initialization of struct.Struct()

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

@@ -1167,6 +1167,15 @@ 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_inspect/test_inspect.py

@@ -6268,7 +6268,8 @@ def test_stat_module_has_signatures(self):
 
  def test_struct_module_has_signatures(self):
  import struct
- self._test_module_has_signatures(struct)
+ unsupported_signature ={'Struct'}
+ self._test_module_has_signatures(struct, unsupported_signature=unsupported_signature)
 
  def test_string_module_has_signatures(self):
  import string

Lib/test/test_struct.py

@@ -582,11 +582,16 @@ 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')
+ self.assertEqual(s.format, 'ii')
+ packed = b'\x01\x00\x00\x00\x02\x00\x00\x00'
+ self.assertEqual(s.pack(1, 2), packed)
+ self.assertEqual(s.unpack(packed), (1, 2))
 
  def check_sizeof(self, format_str, number_of_codes):
  # The size of 'PyStructObject'
- totalsize = support.calcobjsize('2n3P')
+ totalsize = support.calcobjsize('2n3P1?')
  # The size taken up by the 'formatcode' dynamic array
  totalsize += struct.calcsize('P3n0P') * (number_of_codes + 1)
  support.check_sizeof(self, struct.Struct(format_str), totalsize)
@@ -790,7 +795,8 @@ 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):
@@ -824,7 +830,8 @@ def test_endian_table_init_subinterpreters(self):
  self.assertListEqual(list(results), [None] * 5)
 
  def test_operations_on_half_initialized_Struct(self):
- S = struct.Struct.__new__(struct.Struct)
+ with self.assertWarns(DeprecationWarning):
+ S = struct.Struct.__new__(struct.Struct)
 
  spam = array.array('b', b' ')
  self.assertRaises(RuntimeError, S.iter_unpack, spam)

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

@@ -70,6 +70,7 @@ typedef struct{
  formatcode *s_codes;
  PyObject *s_format;
  PyObject *weakreflist; /* List of weak references */
+ bool init_called;
 } PyStructObject;
 
 #define PyStructObject_CAST(op) ((PyStructObject *)(op))
@@ -1757,30 +1758,68 @@ prepare_s(PyStructObject *self)
  return -1;
 }
 
+static int
+s_init(PyStructObject *self, PyObject *format)
+{
+ if (PyUnicode_Check(format)){
+ format = PyUnicode_AsASCIIString(format);
+ if (format == NULL)
+ return -1;
+ }
+ else{
+ Py_INCREF(format);
+ }
+ if (!PyBytes_Check(format)){
+ Py_DECREF(format);
+ PyErr_Format(PyExc_TypeError,
+ "Struct() argument 1 must be a str or bytes object, "
+ "not %T",
+ format);
+ return -1;
+ }
+ Py_SETREF(self->s_format, format);
+ if (prepare_s(self)){
+ return -1;
+ }
+ return 0;
+}
+
 static PyObject *
 s_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 {
-PyObject *self;
+PyStructObject *self;
 
+ if ((PyTuple_GET_SIZE(args) != 1 || kwds)
+ && PyErr_WarnEx(PyExc_DeprecationWarning,
+ "Struct.__new__() has one positional argument", 1))
+{
+ return NULL;
+ }
  assert(type != NULL);
  allocfunc alloc_func = PyType_GetSlot(type, Py_tp_alloc);
  assert(alloc_func != NULL);
-
- self = alloc_func(type, 0);
- if (self != NULL){
- PyStructObject *s = (PyStructObject*)self;
- s->s_format = Py_NewRef(Py_None);
- s->s_codes = NULL;
- s->s_size = -1;
- s->s_len = -1;
+ self = (PyStructObject *)alloc_func(type, 0);
+ if (self == NULL){
+ return NULL;
+ }
+ self->s_format = Py_NewRef(Py_None);
+ self->s_codes = NULL;
+ self->s_size = -1;
+ self->s_len = -1;
+ self->init_called = false;
+ if (PyTuple_GET_SIZE(args) > 0){
+ if (s_init(self, PyTuple_GET_ITEM(args, 0))){
+ Py_DECREF(self);
+ return NULL;
+ }
  }
- return self;
+ return (PyObject *)self;
 }
 
 /*[clinic input]
 Struct.__init__
 
- format: object
+ format: object = NULL
 
 Create a compiled struct object.
 
@@ -1790,32 +1829,28 @@ to the format string. See help(struct) for more on format strings.
 
 static int
 Struct___init___impl(PyStructObject *self, PyObject *format)
-/*[clinic end generated code: output=b8e80862444e92d0 input=1af78a5f57d82cec]*/
+/*[clinic end generated code: output=b8e80862444e92d0 input=6275ff3f85752dd7]*/
 {
- int ret = 0;
-
- if (PyUnicode_Check(format)){
- format = PyUnicode_AsASCIIString(format);
- if (format == NULL)
- return -1;
+ if (!format && !self->s_codes){
+ PyErr_SetString(PyExc_TypeError,
+ "Struct() missing required argument 'format' (pos 1)");
+ return -1;
  }
- else{
- Py_INCREF(format);
+ if (!self->init_called){
+ if (!self->s_codes && s_init(self, format)){
+ return -1;
+ }
+ self->init_called = true;
+ return 0;
  }
-
- if (!PyBytes_Check(format)){
- Py_DECREF(format);
- PyErr_Format(PyExc_TypeError,
- "Struct() argument 1 must be a str or bytes object, "
- "not %.200s",
- _PyType_Name(Py_TYPE(format)));
+ if ((self->s_codes && self->init_called)
+ && PyErr_WarnEx(PyExc_DeprecationWarning,
+ ("Explicit call of __init__() on "
+ "initialized Struct() is deprecated"), 1))
+{
  return -1;
  }
-
- Py_SETREF(self->s_format, format);
-
- ret = prepare_s(self);
- return ret;
+ return s_init(self, format);
 }
 
 static int
@@ -2437,9 +2472,7 @@ static PyType_Slot PyStructType_slots[] ={
 {Py_tp_members, s_members},
 {Py_tp_getset, s_getsetlist},
 {Py_tp_init, Struct___init__},
-{Py_tp_alloc, PyType_GenericAlloc},
 {Py_tp_new, s_new},
-{Py_tp_free, PyObject_GC_Del},
 {0, 0},
 };