gh-67795: Improve precision of non-float timestamp and timeout arguments

Doc/whatsnew/3.15.rst

@@ -281,7 +281,8 @@ Other language changes
 
 * Functions that take timestamp or timeout arguments now accept any real
   numbers (such as :class:`~decimal.Decimal` and :class:`~fractions.Fraction`),
-  not only integers or floats, although this does not improve precision.
+  not only integers or floats.
+  This allows to avoid the precision loss caused by the :class:`float` type.
   (Contributed by Serhiy Storchaka in :gh:`67795`.)
 
 

Include/internal/pycore_global_strings.h

@@ -401,6 +401,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(default)
         STRUCT_FOR_ID(defaultaction)
         STRUCT_FOR_ID(delete)
+        STRUCT_FOR_ID(denominator)
         STRUCT_FOR_ID(depth)
         STRUCT_FOR_ID(desired_access)
         STRUCT_FOR_ID(detect_types)
@@ -643,6 +644,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(nt)
         STRUCT_FOR_ID(null)
         STRUCT_FOR_ID(number)
+        STRUCT_FOR_ID(numerator)
         STRUCT_FOR_ID(obj)
         STRUCT_FOR_ID(object)
         STRUCT_FOR_ID(offset)

Lib/_pydatetime.py

@@ -1857,14 +1857,28 @@ def _fromtimestamp(cls, t, utc, tz):
 
         A timezone info object may be passed in as well.
         """
-        frac, t = _math.modf(t)
-        us = round(frac * 1e6)
-        if us >= 1000000:
+        if isinstance(t, float):
+            frac, t = _math.modf(t)
+            us = round(frac * 1e6)
+        else:
+            try:
+                try:
+                    n, d = t.as_integer_ratio()
+                except AttributeError:
+                    n = t.numerator
+                    d = t.denumerator
+            except AttributeError:
+                frac, t = _math.modf(t)
+                us = round(frac * 1e6)
+            else:
+                t, n = divmod(n, d)
+                us = _divide_and_round(n * 1_000_000, d)
+        if us >= 1_000_000:
             t += 1
-            us -= 1000000
+            us -= 1_000_000
         elif us < 0:
             t -= 1
-            us += 1000000
+            us += 1_000_000
 
         converter = _time.gmtime if utc else _time.localtime
         y, m, d, hh, mm, ss, weekday, jday, dst = converter(t)

Lib/test/datetimetester.py

@@ -2773,11 +2773,11 @@ def utcfromtimestamp(*args, **kwargs):
             self.assertEqual(t, zero)
             t = fts(D('0.000_000_5'))
             self.assertEqual(t, zero)
-            t = fts(D('0.000_000_500_000_000_000_000_1'))
+            t = fts(D('0.000_000_500_000_000_000_000_000_000_000_000_000_001'))
             self.assertEqual(t, one)
             t = fts(D('0.000_000_9'))
             self.assertEqual(t, one)
-            t = fts(D('0.999_999_499_999_999_9'))
+            t = fts(D('0.999_999_499_999_999_999_999_999_999_999_999_999_999'))
             self.assertEqual(t.second, 0)
             self.assertEqual(t.microsecond, 999_999)
             t = fts(D('0.999_999_5'))
@@ -2790,6 +2790,21 @@ def utcfromtimestamp(*args, **kwargs):
             self.assertEqual(t.second, 0)
             self.assertEqual(t.microsecond, 7812)
 
+            t = fts(D('2_147_475_000.000_000_5'))
+            self.assertEqual(t.second, 0)
+            self.assertEqual(t.microsecond, 0)
+            t = fts(D('2_147_475_000'
+                      '.000_000_500_000_000_000_000_000_000_000_000_000_001'))
+            self.assertEqual(t.second, 0)
+            self.assertEqual(t.microsecond, 1)
+            t = fts(D('2_147_475_000'
+                      '.999_999_499_999_999_999_999_999_999_999_999_999_999'))
+            self.assertEqual(t.second, 0)
+            self.assertEqual(t.microsecond, 999_999)
+            t = fts(D('2_147_475_000.999_999_5'))
+            self.assertEqual(t.second, 1)
+            self.assertEqual(t.microsecond, 0)
+
     @support.run_with_tz('MSK-03')  # Something east of Greenwich
     def test_microsecond_rounding_fraction(self):
         F = fractions.Fraction
@@ -2824,11 +2839,13 @@ def utcfromtimestamp(*args, **kwargs):
             self.assertEqual(t, zero)
             t = fts(F(5, 10_000_000))
             self.assertEqual(t, zero)
-            t = fts(F(5_000_000_000, 9_999_999_999_999_999))
+            t = fts(F(        5_000_000_000_000_000_000_000_000_000_000_000,
+                      9_999_999_999_999_999_999_999_999_999_999_999_999_999))
             self.assertEqual(t, one)
             t = fts(F(9, 10_000_000))
             self.assertEqual(t, one)
-            t = fts(F(9_999_995_000_000_000, 10_000_000_000_000_001))
+            t = fts(F( 9_999_995_000_000_000_000_000_000_000_000_000_000_000,
+                      10_000_000_000_000_000_000_000_000_000_000_000_000_001))
             self.assertEqual(t.second, 0)
             self.assertEqual(t.microsecond, 999_999)
             t = fts(F(9_999_995, 10_000_000))
@@ -2841,6 +2858,23 @@ def utcfromtimestamp(*args, **kwargs):
             self.assertEqual(t.second, 0)
             self.assertEqual(t.microsecond, 7812)
 
+            t = fts(2_147_475_000 + F(5, 10_000_000))
+            self.assertEqual(t.second, 0)
+            self.assertEqual(t.microsecond, 0)
+            t = fts(2_147_475_000 +
+                    F(        5_000_000_000_000_000_000_000_000_000_000_000,
+                      9_999_999_999_999_999_999_999_999_999_999_999_999_999))
+            self.assertEqual(t.second, 0)
+            self.assertEqual(t.microsecond, 1)
+            t = fts(2_147_475_000 +
+                    F( 9_999_995_000_000_000_000_000_000_000_000_000_000_000,
+                      10_000_000_000_000_000_000_000_000_000_000_000_000_001))
+            self.assertEqual(t.second, 0)
+            self.assertEqual(t.microsecond, 999_999)
+            t = fts(2_147_475_000 + F(9_999_995, 10_000_000))
+            self.assertEqual(t.second, 1)
+            self.assertEqual(t.microsecond, 0)
+
     def test_timestamp_limits(self):
         with self.subTest("minimum UTC"):
             min_dt = self.theclass.min.replace(tzinfo=timezone.utc)

Lib/test/test_os.py

@@ -951,16 +951,12 @@ def ns_to_sec(ns):
     @staticmethod
     def ns_to_sec_decimal(ns):
         # Convert a number of nanosecond (int) to a number of seconds (Decimal).
-        # Round towards infinity by adding 0.5 nanosecond to avoid rounding
-        # issue, os.utime() rounds towards minus infinity.
-        return decimal.Decimal('1e-9') * ns + decimal.Decimal('0.5e-9')
+        return decimal.Decimal('1e-9') * ns
 
     @staticmethod
     def ns_to_sec_fraction(ns):
         # Convert a number of nanosecond (int) to a number of seconds (Fraction).
-        # Round towards infinity by adding 0.5 nanosecond to avoid rounding
-        # issue, os.utime() rounds towards minus infinity.
-        return fractions.Fraction(ns, 10**9) + fractions.Fraction(1, 2*10**9)
+        return fractions.Fraction(ns, 10**9)
 
     def test_utime_by_indexed(self):
         # pass times as floating-point seconds as the second indexed parameter

Misc/NEWS.d/next/Library/2025-09-22-11-30-45.gh-issue-67795.fROoZt.rst

@@ -1,3 +1,4 @@
 Functions that take timestamp or timeout arguments now accept any real
 numbers (such as :class:`~decimal.Decimal` and :class:`~fractions.Fraction`),
-not only integers or floats, although this does not improve precision.
+not only integers or floats.
+This allows to avoid the precision loss caused by the :class:`float` type.