[Python-Dev] PEP 410 (Decimal timestamp): the implementation is ready for a review (original) (raw)

Victor Stinner victor.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 (220 ~= 106). A 1 ms precision loss looks like a bug.

Oh... It was a important bug in my function used to change the denominator of a timestamp. I tried to workaround integer overflow, but I added a bug. I changed my patch to use PyLong which has no integer overflow issue.

Fixed example:

open("test", "x").close() import shutil shutil.copy2("test", "test2") [94386 refs] print(os.stat("test", datetime.datetime).stmtime) 2012-02-16 21:58:30.835062+00:00 print(os.stat("test2", datetime.datetime).stmtime) 2012-02-16 21:58:30.835062+00:00 print(os.stat("test", decimal.Decimal).stmtime) 1329429510.835061686 print(os.stat("test2", decimal.Decimal).stmtime) 1329429510.835061789 os.stat("test2", decimal.Decimal).stmtime - os.stat("test", decimal.Decimal).stmtime Decimal('1.03E-7')

So the difference is only 0.1 us (100 ns).

It doesn't change anything to the Makefile issue, if timestamps are different in a single nanosecond, they are seen as different by make (by another program comparing the timestamp of two files using nanosecond precision).

Victor

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