Issue #23517: fromtimestamp() and utcfromtimestamp() methods of

datetime.datetime now round microseconds to nearest with ties going to nearest
even integer (ROUND_HALF_EVEN), as round(float), instead of rounding towards
-Infinity (ROUND_FLOOR).

pytime API: replace _PyTime_ROUND_HALF_UP with _PyTime_ROUND_HALF_EVEN. Fix
also _PyTime_Divide() for negative numbers.

_PyTime_AsTimeval_impl() now reuses _PyTime_Divide() instead of reimplementing
rounding modes.

Author: vstinner

Include/pytime.h

@@ -31,9 +31,9 @@ typedef enum {
     /* Round towards infinity (+inf).
        For example, used for timeout to wait "at least" N seconds. */
     _PyTime_ROUND_CEILING=1,
-    /* Round to nearest with ties going away from zero.
+    /* Round to nearest with ties going to nearest even integer.
        For example, used to round from a Python float. */
-    _PyTime_ROUND_HALF_UP
+    _PyTime_ROUND_HALF_EVEN
 } _PyTime_round_t;
 
 /* Convert a time_t to a PyLong. */
@@ -44,8 +44,9 @@ PyAPI_FUNC(PyObject *) _PyLong_FromTime_t(
 PyAPI_FUNC(time_t) _PyLong_AsTime_t(
     PyObject *obj);
 
-/* Round to nearest with ties going away from zero (_PyTime_ROUND_HALF_UP). */
-PyAPI_FUNC(double) _PyTime_RoundHalfUp(
+/* Round to nearest with ties going to nearest even integer
+   (_PyTime_ROUND_HALF_EVEN) */
+PyAPI_FUNC(double) _PyTime_RoundHalfEven(
     double x);
 
 /* Convert a number of seconds, int or float, to time_t. */

Lib/datetime.py

@@ -1380,7 +1380,7 @@ def _fromtimestamp(cls, t, utc, tz):
         A timezone info object may be passed in as well.
         """
         frac, t = _math.modf(t)
-        us = _round_half_up(frac * 1e6)
+        us = round(frac * 1e6)
         if us >= 1000000:
             t += 1
             us -= 1000000

Lib/test/datetimetester.py

@@ -1874,7 +1874,7 @@ def test_microsecond_rounding(self):
                 self.assertEqual(t, zero)
                 t = fts(-1/2**7)
                 self.assertEqual(t.second, 59)
-                self.assertEqual(t.microsecond, 992187)
+                self.assertEqual(t.microsecond, 992188)
 
             t = fts(1e-7)
             self.assertEqual(t, zero)
@@ -1888,7 +1888,7 @@ def test_microsecond_rounding(self):
             self.assertEqual(t.microsecond, 0)
             t = fts(1/2**7)
             self.assertEqual(t.second, 0)
-            self.assertEqual(t.microsecond, 7813)
+            self.assertEqual(t.microsecond, 7812)
 
     def test_insane_fromtimestamp(self):
         # It's possible that some platform maps time_t to double,