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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.
This commit is contained in:
Victor Stinner 2015-09-18 14:42:05 +02:00
parent e3bcbd2bba
commit 511491ade0
4 changed files with 114 additions and 42 deletions
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53
Lib/datetime.py
View File

@ -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

24
Lib/test/datetimetester.py
View File

@ -650,8 +650,16 @@ class TestTimeDelta(HarmlessMixedComparison, unittest.TestCase):
# Single-field rounding.
eq(td(milliseconds=0.4/1000), td(0)) # rounds to 0
eq(td(milliseconds=-0.4/1000), td(0)) # rounds to 0
eq(td(milliseconds=0.5/1000), td(microseconds=0))
eq(td(milliseconds=-0.5/1000), td(microseconds=-0))
eq(td(milliseconds=0.6/1000), td(microseconds=1))
eq(td(milliseconds=-0.6/1000), td(microseconds=-1))
eq(td(milliseconds=1.5/1000), td(microseconds=2))
eq(td(milliseconds=-1.5/1000), td(microseconds=-2))
eq(td(seconds=0.5/10**6), td(microseconds=0))
eq(td(seconds=-0.5/10**6), td(microseconds=-0))
eq(td(seconds=1/2**7), td(microseconds=7812))
eq(td(seconds=-1/2**7), td(microseconds=-7812))
# Rounding due to contributions from more than one field.
us_per_hour = 3600e6
@ -1824,12 +1832,14 @@ class TestDateTime(TestDate):
tzinfo=timezone(timedelta(hours=-5), 'EST'))
self.assertEqual(t.timestamp(),
18000 + 3600 + 2*60 + 3 + 4*1e-6)
def test_microsecond_rounding(self):
for fts in [self.theclass.fromtimestamp,
self.theclass.utcfromtimestamp]:
zero = fts(0)
self.assertEqual(zero.second, 0)
self.assertEqual(zero.microsecond, 0)
one = fts(1e-6)
try:
minus_one = fts(-1e-6)
except OSError:
@ -1840,22 +1850,28 @@ class TestDateTime(TestDate):
self.assertEqual(minus_one.microsecond, 999999)
t = fts(-1e-8)
self.assertEqual(t, minus_one)
self.assertEqual(t, zero)
t = fts(-9e-7)
self.assertEqual(t, minus_one)
t = fts(-1e-7)
self.assertEqual(t, minus_one)
self.assertEqual(t, zero)
t = fts(-1/2**7)
self.assertEqual(t.second, 59)
self.assertEqual(t.microsecond, 992188)
t = fts(1e-7)
self.assertEqual(t, zero)
t = fts(9e-7)
self.assertEqual(t, zero)
self.assertEqual(t, one)
t = fts(0.99999949)
self.assertEqual(t.second, 0)
self.assertEqual(t.microsecond, 999999)
t = fts(0.9999999)
self.assertEqual(t.second, 1)
self.assertEqual(t.microsecond, 0)
t = fts(1/2**7)
self.assertEqual(t.second, 0)
self.assertEqual(t.microsecond, 999999)
self.assertEqual(t.microsecond, 7812)
def test_insane_fromtimestamp(self):
# It's possible that some platform maps time_t to double,

8
Misc/NEWS
View File

@ -81,6 +81,14 @@ Core and Builtins
Library
-------
- 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.
- Issue #24684: socket.socket.getaddrinfo() now calls
PyUnicode_AsEncodedString() instead of calling the encode() method of the
host, to handle correctly custom string with an encode() method which doesn't

71
Modules/_datetimemodule.c
View File

@ -4113,6 +4113,44 @@ datetime_from_timet_and_us(PyObject *cls, TM_FUNC f, time_t timet, int us,
tzinfo);
}
static time_t
_PyTime_DoubleToTimet(double x)
{
time_t result;
double diff;
result = (time_t)x;
/* How much info did we lose? time_t may be an integral or
* floating type, and we don't know which. If it's integral,
* we don't know whether C truncates, rounds, returns the floor,
* etc. If we lost a second or more, the C rounding is
* unreasonable, or the input just doesn't fit in a time_t;
* call it an error regardless. Note that the original cast to
* time_t can cause a C error too, but nothing we can do to
* worm around that.
*/
diff = x - (double)result;
if (diff <= -1.0 || diff >= 1.0) {
PyErr_SetString(PyExc_OverflowError,
"timestamp out of range for platform time_t");
result = (time_t)-1;
}
return result;
}
/* Round a double to the nearest long. |x| must be small enough to fit
* in a C long; this is not checked.
*/
static double
_PyTime_RoundHalfEven(double x)
{
double rounded = round(x);
if (fabs(x-rounded) == 0.5)
/* halfway case: round to even */
rounded = 2.0*round(x/2.0);
return rounded;
}
/* Internal helper.
* Build datetime from a Python timestamp. Pass localtime or gmtime for f,
* to control the interpretation of the timestamp. Since a double doesn't
@ -4121,15 +4159,32 @@ datetime_from_timet_and_us(PyObject *cls, TM_FUNC f, time_t timet, int us,
* to get that much precision (e.g., C time() isn't good enough).
*/
static PyObject *
datetime_from_timestamp(PyObject *cls, TM_FUNC f, PyObject *timestamp,
datetime_from_timestamp(PyObject *cls, TM_FUNC f, double timestamp,
PyObject *tzinfo)
{
time_t timet;
long us;
double fraction;
int us;
if (_PyTime_ObjectToTimeval(timestamp, &timet, &us, _PyTime_ROUND_DOWN) == -1)
timet = _PyTime_DoubleToTimet(timestamp);
if (timet == (time_t)-1 && PyErr_Occurred())
return NULL;
return datetime_from_timet_and_us(cls, f, timet, (int)us, tzinfo);
fraction = timestamp - (double)timet;
us = (int)_PyTime_RoundHalfEven(fraction * 1e6);
if (us < 0) {
/* Truncation towards zero is not what we wanted
for negative numbers (Python's mod semantics) */
timet -= 1;
us += 1000000;
}
/* If timestamp is less than one microsecond smaller than a
* full second, round up. Otherwise, ValueErrors are raised
* for some floats. */
if (us == 1000000) {
timet += 1;
us = 0;
}
return datetime_from_timet_and_us(cls, f, timet, us, tzinfo);
}
/* Internal helper.
@ -4231,11 +4286,11 @@ static PyObject *
datetime_fromtimestamp(PyObject *cls, PyObject *args, PyObject *kw)
{
PyObject *self;
PyObject *timestamp;
double timestamp;
PyObject *tzinfo = Py_None;
static char *keywords[] = {"timestamp", "tz", NULL};
if (! PyArg_ParseTupleAndKeywords(args, kw, "O|O:fromtimestamp",
if (! PyArg_ParseTupleAndKeywords(args, kw, "d|O:fromtimestamp",
keywords, &timestamp, &tzinfo))
return NULL;
if (check_tzinfo_subclass(tzinfo) < 0)
@ -4259,10 +4314,10 @@ datetime_fromtimestamp(PyObject *cls, PyObject *args, PyObject *kw)
static PyObject *
datetime_utcfromtimestamp(PyObject *cls, PyObject *args)
{
PyObject *timestamp;
double timestamp;
PyObject *result = NULL;
if (PyArg_ParseTuple(args, "O:utcfromtimestamp", &timestamp))
if (PyArg_ParseTuple(args, "d:utcfromtimestamp", &timestamp))
result = datetime_from_timestamp(cls, gmtime, timestamp,
Py_None);
return result;