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gh-121249: Deprecate using F/D type codes in the struct module #152309

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vstinner merged 6 commits into from
Jun 30, 2026
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gh-121249: Deprecate using F/D type codes in the struct module #152309

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gh-121249: Deprecate using F/D type codes in the struct module
skirpichev Jun 26, 2026
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adjust test
skirpichev Jun 26, 2026
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skirpichev Jun 27, 2026
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skirpichev Jun 30, 2026
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address review
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skirpichev Jun 30, 2026
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6 changes: 6 additions & 0 deletions Doc/deprecations/pending-removal-in-3.21.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -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
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two-letter forms ``'Zf'`` and ``'Zd'``.
7 changes: 7 additions & 0 deletions Doc/whatsnew/3.16.rst
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Expand Up @@ -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
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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
Expand Down
12 changes: 6 additions & 6 deletions Lib/test/test_ctypes/test_byteswap.py
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Expand Up @@ -178,14 +178,14 @@ def test_endian_float_complex(self):
self.assertIs(c_float_complex.__ctype_le__.__ctype_be__,
c_float_complex)
s = c_float_complex(math.pi+1j)
self.assertEqual(bin(struct.pack("F", math.pi+1j)), bin(s))
self.assertEqual(bin(struct.pack("Zf", math.pi+1j)), bin(s))
self.assertAlmostEqual(s.value, math.pi+1j, places=6)
s = c_float_complex.__ctype_le__(math.pi+1j)
self.assertAlmostEqual(s.value, math.pi+1j, places=6)
self.assertEqual(bin(struct.pack("<F", math.pi+1j)), bin(s))
self.assertEqual(bin(struct.pack("<Zf", math.pi+1j)), bin(s))
s = c_float_complex.__ctype_be__(math.pi+1j)
self.assertAlmostEqual(s.value, math.pi+1j, places=6)
self.assertEqual(bin(struct.pack(">F", math.pi+1j)), bin(s))
self.assertEqual(bin(struct.pack(">Zf", math.pi+1j)), bin(s))

@unittest.skipUnless(hasattr(ctypes, 'c_double_complex'), "No complex types")
def test_endian_double_complex(self):
Expand All @@ -199,14 +199,14 @@ def test_endian_double_complex(self):
self.assertIs(c_double_complex.__ctype_le__.__ctype_be__,
c_double_complex)
s = c_double_complex(math.pi+1j)
self.assertEqual(bin(struct.pack("D", math.pi+1j)), bin(s))
self.assertEqual(bin(struct.pack("Zd", math.pi+1j)), bin(s))
self.assertAlmostEqual(s.value, math.pi+1j, places=6)
s = c_double_complex.__ctype_le__(math.pi+1j)
self.assertAlmostEqual(s.value, math.pi+1j, places=6)
self.assertEqual(bin(struct.pack("<D", math.pi+1j)), bin(s))
self.assertEqual(bin(struct.pack("<Zd", math.pi+1j)), bin(s))
s = c_double_complex.__ctype_be__(math.pi+1j)
self.assertAlmostEqual(s.value, math.pi+1j, places=6)
self.assertEqual(bin(struct.pack(">D", math.pi+1j)), bin(s))
self.assertEqual(bin(struct.pack(">Zd", math.pi+1j)), bin(s))

def test_endian_other(self):
self.assertIs(c_byte.__ctype_le__, c_byte)
Expand Down
4 changes: 2 additions & 2 deletions Lib/test/test_memoryview.py
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Expand Up @@ -717,8 +717,8 @@ def test_half_float(self):
self.assertListEqual(half_view.tolist(), float_view.tolist())

def test_complex_types(self):
float_complex_data = struct.pack('FFF', 0.0, -1.5j, 1+2j)
double_complex_data = struct.pack('DDD', 0.0, -1.5j, 1+2j)
float_complex_data = struct.pack('ZfZfZf', 0.0, -1.5j, 1+2j)
double_complex_data = struct.pack('ZdZdZd', 0.0, -1.5j, 1+2j)
float_complex_view = memoryview(float_complex_data).cast('Zf')
double_complex_view = memoryview(double_complex_data).cast('Zd')
self.assertEqual(float_complex_view.nbytes * 2, double_complex_view.nbytes)
Expand Down
22 changes: 17 additions & 5 deletions Lib/test/test_struct.py
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Expand Up @@ -7,6 +7,7 @@
import unittest
import struct
import sys
import warnings
import weakref

from test import support
Expand Down Expand Up @@ -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):
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with warnings.catch_warnings():
warnings.simplefilter("error", DeprecationWarning)
self.assertRaises(DeprecationWarning, struct.pack, f, z)

@vstinner vstinner Jun 29, 2026

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We already check with self.assertWarns(DeprecationWarning): below. I'm not sure that checking for DeprecationWarning exception is useful.

@skirpichev skirpichev Jun 30, 2026

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We do this in other places.

with warnings.catch_warnings():
with self.assertWarns(DeprecationWarning):
b = struct.pack(f, z)
with warnings.catch_warnings():
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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,
Expand Down
1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2026-06-26-19-01-13.gh-issue-121249.p1SBW0.rst
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@@ -0,0 +1 @@
Deprecate using ``'F'`` and ``'D'`` type codes in the :mod:`struct`.
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29 changes: 29 additions & 0 deletions Modules/_struct.c
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Expand Up @@ -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':
Expand Down Expand Up @@ -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)
Expand Down
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