Issue #23517: Fix rounding in fromtimestamp() and utcfromtimestamp() methods

of datetime.datetime: microseconds are now rounded to nearest with ties going
to nearest even integer (ROUND_HALF_EVEN), instead of being rounding towards
zero (ROUND_DOWN). It's important that these methods use the same rounding
mode than datetime.timedelta to keep the property:

   (datetime(1970,1,1) + timedelta(seconds=t)) == datetime.utcfromtimestamp(t)

It also the rounding mode used by round(float) for example.

Add more unit tests on the rounding mode in test_datetime.

Lib/datetime.py

@@ -1361,50 +1361,43 @@ class datetime(date):
         """timezone info object"""
         return self._tzinfo
 
+    @classmethod
+    def _fromtimestamp(cls, t, utc, tz):
+        """Construct a datetime from a POSIX timestamp (like time.time()).
+
+        A timezone info object may be passed in as well.
+        """
+        frac, t = _math.modf(t)
+        us = round(frac * 1e6)
+        if us >= 1000000:
+            t += 1
+            us -= 1000000
+        elif us < 0:
+            t -= 1
+            us += 1000000
+
+        converter = _time.gmtime if utc else _time.localtime
+        y, m, d, hh, mm, ss, weekday, jday, dst = converter(t)
+        ss = min(ss, 59)    # clamp out leap seconds if the platform has them
+        return cls(y, m, d, hh, mm, ss, us, tz)
+
     @classmethod
     def fromtimestamp(cls, t, tz=None):
         """Construct a datetime from a POSIX timestamp (like time.time()).
 
         A timezone info object may be passed in as well.
         """
-
         _check_tzinfo_arg(tz)
 
-        converter = _time.localtime if tz is None else _time.gmtime
-
-        t, frac = divmod(t, 1.0)
-        us = int(frac * 1e6)
-
-        # If timestamp is less than one microsecond smaller than a
-        # full second, us can be rounded up to 1000000.  In this case,
-        # roll over to seconds, otherwise, ValueError is raised
-        # by the constructor.
-        if us == 1000000:
-            t += 1
-            us = 0
-        y, m, d, hh, mm, ss, weekday, jday, dst = converter(t)
-        ss = min(ss, 59)    # clamp out leap seconds if the platform has them
-        result = cls(y, m, d, hh, mm, ss, us, tz)
+        result = cls._fromtimestamp(t, tz is not None, tz)
         if tz is not None:
             result = tz.fromutc(result)
         return result
 
     @classmethod
     def utcfromtimestamp(cls, t):
-        "Construct a UTC datetime from a POSIX timestamp (like time.time())."
-        t, frac = divmod(t, 1.0)
-        us = int(frac * 1e6)
-
-        # If timestamp is less than one microsecond smaller than a
-        # full second, us can be rounded up to 1000000.  In this case,
-        # roll over to seconds, otherwise, ValueError is raised
-        # by the constructor.
-        if us == 1000000:
-            t += 1
-            us = 0
-        y, m, d, hh, mm, ss, weekday, jday, dst = _time.gmtime(t)
-        ss = min(ss, 59)    # clamp out leap seconds if the platform has them
-        return cls(y, m, d, hh, mm, ss, us)
+        """Construct a naive UTC datetime from a POSIX timestamp."""
+        return cls._fromtimestamp(t, True, None)
 
     # XXX This is supposed to do better than we *can* do by using time.time(),
     # XXX if the platform supports a more accurate way.  The C implementation