gh-85989: deprecate float.__getformat__() class method

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

@@ -1,6 +1,12 @@
 Pending removal in Python 3.21
 ------------------------------
 
+* The ``__getformat__()`` class method of the :class:`float` is deprecated and
+  will be removed in Python 3.21.  On CPython, ``float.__getformat__()`` always
+  return a string, prefixed with ``"IEEE"``: to build CPython, you need support
+  for IEEE 754 floating-point numbers since Python 3.11.  (Contributed by
+  Sergey B Kirpichev in :gh:`85989`.)
+
 * :mod:`abc`
 
    * Soft-deprecated since Python 3.3 :class:`abc.abstractclassmethod`,

Doc/whatsnew/3.16.rst

@@ -736,6 +736,12 @@ Deprecated
 New deprecations
 ----------------
 
+* The ``__getformat__()`` class method of the :class:`float` is deprecated
+  and will be removed in Python 3.21.  On CPython, ``float.__getformat__()``
+  always return a string, prefixed with ``"IEEE"``: to build CPython, you
+  need support for IEEE 754 floating-point numbers since Python 3.11.
+  (Contributed by Sergey B Kirpichev in :gh:`85989`.)
+
 * :mod:`abc`
 
   * Soft-deprecated since Python 3.3 :class:`abc.abstractclassmethod`,

Lib/test/pythoninfo.py

@@ -220,11 +220,6 @@ def collect_locale(info_add):
     info_add('locale.getencoding', locale.getencoding())
 
 
-def collect_builtins(info_add):
-    info_add('builtins.float.float_format', float.__getformat__("float"))
-    info_add('builtins.float.double_format', float.__getformat__("double"))
-
-
 def collect_urandom(info_add):
     import os
 
@@ -1333,7 +1328,6 @@ def collect_info(info):
         # its state.
         collect_urandom,
 
-        collect_builtins,
         collect_cc,
         collect_curses,
         collect_datetime,

Lib/test/support/__init__.py

@@ -532,10 +532,44 @@ def dec(*args, **kwargs):
 # for a discussion of this number.
 SOCK_MAX_SIZE = 16 * 1024 * 1024 + 1
 
+# This helper exists for alternative Python implementations, that use
+# the CPython test suite.
+def _have_ieee_doubles():
+    if sys.implementation.name == 'cpython':
+        return True
+    import math
+    import struct
+    # Check parameters for encoding of floats; a quick exit
+    # if they aren't same as for IEC 60559 doubles.  Check
+    # also that subnormals are present.
+    if (struct.calcsize('d') != 8
+            or sys.float_info.radix != 2
+            or sys.float_info.mant_dig != 53
+            or sys.float_info.dig != 15
+            or sys.float_info.min_exp != -1021
+            or sys.float_info.min_10_exp != -307
+            or sys.float_info.max_exp != 1024
+            or sys.float_info.max_10_exp != 308
+            or not math.issubnormal(math.nextafter(0, 1))):
+        return False
+    # We attempt to determine if this machine is using IEC
+    # floating-point formats by peering at the bits of some
+    # carefully chosen value.  Assume that integer and
+    # floating-point types have same endianness.
+    d = 9006104071832581.0
+    d_be_bytes = b"\x43\x3f\xff\x01\x02\x03\x04\x05"
+    d_packed = struct.pack('d', d)
+    if sys.byteorder == 'little':
+        return d_packed == bytes(reversed(d_be_bytes))
+    return d_packed == d_be_bytes
+
+HAVE_IEEE_754 = _have_ieee_doubles()
+
 # decorator for skipping tests on non-IEEE 754 platforms
 requires_IEEE_754 = unittest.skipUnless(
-    float.__getformat__("double").startswith("IEEE"),
+    HAVE_IEEE_754,
     "test requires IEEE 754 doubles")
+del HAVE_IEEE_754
 
 # detect evidence of double-rounding:
 x, y = 1e16, 2.9999 # use temporary values to defeat peephole optimizer

Lib/test/test_capi/test_float.py

@@ -7,6 +7,7 @@
 from test.test_capi.test_getargs import (Float, FloatSubclass, FloatSubclass2,
                                          BadIndex2, BadFloat2, Index, BadIndex,
                                          BadFloat)
+from test import support
 from test.support import import_helper
 
 _testcapi = import_helper.import_module('_testcapi')
@@ -23,7 +24,6 @@
     8: 2.0 ** -53,  # binary64
 }
 
-HAVE_IEEE_754 = float.__getformat__("double").startswith("IEEE")
 INF = float("inf")
 NAN = float("nan")
 
@@ -170,14 +170,13 @@ def test_unpack(self):
         self.assertEqual(unpack(b'\x00\x00\x00\x00\x00\x00\xf8?', LITTLE_ENDIAN),
                          1.5)
 
+    @support.requires_IEEE_754
     def test_pack_unpack_roundtrip(self):
         pack = _testcapi.float_pack
         unpack = _testcapi.float_unpack
 
         large = 2.0 ** 100
-        values = [1.0, 1.5, large, 1.0/7, math.pi]
-        if HAVE_IEEE_754:
-            values.extend((INF, NAN))
+        values = [1.0, 1.5, large, 1.0/7, math.pi, INF, NAN]
         for value in values:
             for size in (2, 4, 8,):
                 if size == 2 and value == large:
@@ -196,7 +195,7 @@ def test_pack_unpack_roundtrip(self):
                         else:
                             self.assertEqual(value2, value)
 
-    @unittest.skipUnless(HAVE_IEEE_754, "requires IEEE 754")
+    @support.requires_IEEE_754
     def test_pack_unpack_roundtrip_for_nans(self):
         pack = _testcapi.float_pack
         unpack = _testcapi.float_unpack
@@ -228,7 +227,7 @@ def test_pack_unpack_roundtrip_for_nans(self):
                         self.assertTrue(math.isnan(value))
                         self.assertEqual(data1, data2)
 
-    @unittest.skipUnless(HAVE_IEEE_754, "requires IEEE 754")
+    @support.requires_IEEE_754
     @unittest.skipUnless(sys.maxsize != 2147483647, "requires 64-bit mode")
     def test_pack_unpack_nans_for_different_formats(self):
         pack = _testcapi.float_pack

Lib/test/test_float.py

@@ -673,12 +673,13 @@ def __neg__(self):
 @unittest.skipUnless(hasattr(float, "__getformat__"), "requires __getformat__")
 class FormatFunctionsTestCase(unittest.TestCase):
     def test_getformat(self):
-        self.assertIn(float.__getformat__('double'),
-                      ['unknown', 'IEEE, big-endian', 'IEEE, little-endian'])
-        self.assertIn(float.__getformat__('float'),
-                      ['unknown', 'IEEE, big-endian', 'IEEE, little-endian'])
-        self.assertRaises(ValueError, float.__getformat__, 'chicken')
-        self.assertRaises(TypeError, float.__getformat__, 1)
+        with self.assertWarns(DeprecationWarning):
+            self.assertIn(float.__getformat__('double'),
+                          ['unknown', 'IEEE, big-endian', 'IEEE, little-endian'])
+            self.assertIn(float.__getformat__('float'),
+                          ['unknown', 'IEEE, big-endian', 'IEEE, little-endian'])
+            self.assertRaises(ValueError, float.__getformat__, 'chicken')
+            self.assertRaises(TypeError, float.__getformat__, 1)
 
 
 BE_DOUBLE_INF = b'\x7f\xf0\x00\x00\x00\x00\x00\x00'

Lib/test/test_funcattrs.py

@@ -486,8 +486,7 @@ def test_builtin__qualname__(self):
 
         # builtin classmethod:
         self.assertEqual(dict.fromkeys.__qualname__, 'dict.fromkeys')
-        self.assertEqual(float.__getformat__.__qualname__,
-                         'float.__getformat__')
+        self.assertEqual(int.from_bytes.__qualname__, 'int.from_bytes')
 
         # builtin staticmethod:
         self.assertEqual(str.maketrans.__qualname__, 'str.maketrans')
@@ -509,7 +508,7 @@ def test_builtin__self__(self):
 
         # builtin classmethod:
         self.assertIs(dict.fromkeys.__self__, dict)
-        self.assertIs(float.__getformat__.__self__, float)
+        self.assertIs(int.from_bytes.__self__, int)
 
         # builtin staticmethod:
         self.assertIsNone(str.maketrans.__self__)

Misc/NEWS.d/next/Core_and_Builtins/2026-03-24-17-29-19.gh-issue-145633.RtktKR.rst

@@ -0,0 +1,4 @@
+The ``__getformat__()`` class method of the :class:`float` is deprecated.  On
+CPython, ``float.__getformat__()`` always return a string, prefixed with
+``"IEEE"``: to build CPython, you need support for IEEE 754 floating-point
+numbers since Python 3.11.  Patch by Sergey B Kirpichev.

Objects/floatobject.c

@@ -1690,6 +1690,11 @@ static PyObject *
 float___getformat___impl(PyTypeObject *type, const char *typestr)
 /*[clinic end generated code: output=2bfb987228cc9628 input=eb1cf45e9bddab72]*/
 {
+    if (PyErr_WarnFormat(PyExc_DeprecationWarning, 1,
+                         "float.__getformat__() is deprecated"))
+    {
+        return NULL;
+    }
     if (strcmp(typestr, "double") != 0 && strcmp(typestr, "float") != 0) {
         PyErr_SetString(PyExc_ValueError,
                         "__getformat__() argument 1 must be "