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[Python-Dev] PEP 410 (Decimal timestamp): the implementation is ready for a review
Victor Stinnervictor.stinner at gmail.com
Thu Feb 16 23:04:41 CET 2012
>> > $ stat test | \grep Modify>> > Modify: 2012-02-16 13:51:25.643597139 +0100>> > $ stat test2 | \grep Modify>> > Modify: 2012-02-16 13:51:25.643597126 +0100>>>> The loss of precision is not constant: it depends on the timestamp value.>> Well, I've tried several times and I can't reproduce a 1 ms difference.>>> The loss of precision is between 1 ms and 4 us.>> It still looks fishy to me. IEEE doubles have a 52-bit mantissa. Since> the integral part of a timestamp takes 32 bits or less, there are still> 20 bits left for the fractional part: which allows for at least a 1 µs> precision (2**20 ~= 10**6). A 1 ms precision loss looks like a bug.Oh... It was a important bug in my function used to change thedenominator of a timestamp. I tried to workaround integer overflow,but I added a bug. I changed my patch to use PyLong which has nointeger overflow issue.Fixed example:>>> open("test", "x").close()>>> import shutil>>> shutil.copy2("test", "test2")[94386 refs]>>> print(os.stat("test", datetime.datetime).st_mtime)2012-02-16 21:58:30.835062+00:00>>> print(os.stat("test2", datetime.datetime).st_mtime)2012-02-16 21:58:30.835062+00:00>>> print(os.stat("test", decimal.Decimal).st_mtime)1329429510.835061686>>> print(os.stat("test2", decimal.Decimal).st_mtime)1329429510.835061789>>> os.stat("test2", decimal.Decimal).st_mtime - os.stat("test", decimal.Decimal).st_mtimeDecimal('1.03E-7')So the difference is only 0.1 us (100 ns).It doesn't change anything to the Makefile issue, if timestamps aredifferent in a single nanosecond, they are seen as different by make(by another program comparing the timestamp of two files usingnanosecond precision).Victor
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