[Python-Dev] How to round timestamps and durations? (original) (raw)

Victor Stinner victor.stinner at gmail.com
Mon Feb 13 13:59:18 CET 2012

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

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