[Python-Dev] How to round timestamps and durations?

[Victor Stinner]

Hi,

My work on the PEP 410 tries to unify the code to manipulate
timestamps. The problem is that I'm unable to decide how to round
these numbers.

Functions using a resolution of 1 second (e.g. time.mktime) expects
rounding towards zero (ROUND_HALF_DOWN), as does int(float). Example:

>>> time.mktime(time.localtime(-1.9)), time.mktime(time.localtime(1.9))
(-1.0, 1.0)

datetime.datetime.fromtimestamp() rounds to nearest with ties going
away from zero (ROUND_HALF_UP). Example:

>>> datetime.datetime.fromtimestamp(-1.1e-6), datetime.datetime.fromtimestamp(1.1e-6)
(datetime.datetime(1970, 1, 1, 0, 59, 59, 999999),
datetime.datetime(1970, 1, 1, 1, 0, 0, 1))
>>> datetime.datetime.fromtimestamp(-1.9e-6), datetime.datetime.fromtimestamp(1.9e-6)
(datetime.datetime(1970, 1, 1, 0, 59, 59, 999998),
datetime.datetime(1970, 1, 1, 1, 0, 0, 2))

datetime.timedelta * float  and datetime.timedelta / float rounds to
nearest with ties going to nearest even integer (ROUND_HALF_EVEN), as
does round(). Example:

>>> [(datetime.timedelta(microseconds=x) / 2.0).microseconds for x in range(6)]
[0, 0, 1, 2, 2, 2]

Should I also support multiple rounding methods depending on the
operation and of the Python function? Should we always use the same
rounding method?

Antoine pointed me that ROUND_HALF_UP can produce timestamps "in the
future", which is especially visible when using a resolution of 1
second. I like this rounding method because it limits the loss of
precision to an half unit: abs(rounded - timestamp) <= 0.5. But it can
be "surprising".

The rounding method should maybe be the same than int(float) (so
ROUND_HALF_DOWN) to avoid surprising results for applications using
int(time.time()) for example (I had such problem with rotated logs and
test_logging).

--

There is an issue on rounding timedelta:
http://bugs.python.org/issue8860

Victor