python · serhiy-storchaka · Sep 23, 2025 · Oct 3, 2025 · Oct 3, 2025 · May 4, 2026
@@ -568,7 +568,8 @@ 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`.)

 .. _whatsnew315-bytearray-take-bytes:

diff --git a/Include/internal/pycore_global_objects_fini_generated.h b/Include/internal/pycore_global_objects_fini_generated.h
diff --git a/Include/internal/pycore_global_strings.h b/Include/internal/pycore_global_strings.h
@@ -424,6 +424,7 @@ 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)
@@ -674,6 +675,7 @@ 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)

diff --git a/Include/internal/pycore_runtime_init_generated.h b/Include/internal/pycore_runtime_init_generated.h
diff --git a/Include/internal/pycore_unicodeobject_generated.h b/Include/internal/pycore_unicodeobject_generated.h
@@ -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)

@@ -2802,11 +2802,11 @@ 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'))
@@ -2819,6 +2819,21 @@ 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
@@ -2849,11 +2864,13 @@ 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))
@@ -2866,6 +2883,23 @@ 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)

diff --git a/Lib/test/test_os/test_os.py b/Lib/test/test_os/test_os.py
@@ -1110,16 +1110,12 @@ 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

diff --git a/Misc/NEWS.d/next/Library/2026-05-04-20-10-04.gh-issue-67795.v9wb5q.rst b/Misc/NEWS.d/next/Library/2026-05-04-20-10-04.gh-issue-67795.v9wb5q.rst
@@ -0,0 +1,3 @@
+Avoid the precision loss caused by the :class:`float` type in functions that
+take more precise numbers (such as :class:`~decimal.Decimal` and
+:class:`~fractions.Fraction`) as timestamp or timeout arguments.