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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 13:46:18 CET 2012
> A data point on this specific use case. The following code throws its> assert ~90% of the time in Python 3.2.2 on a modern Linux machine (assuming> "foo" exists and "bar" does not):>> import shutil> import os> shutil.copy2("foo", "bar")> assert os.stat("foo").st_mtime == os.stat("bar").st_mtimeIt works because Python uses float for utime() and for stat(). Butthis assertion may fail if another program checks file timestampswithout lossing precision (because of float), e.g. a program writtenin C that compares st_*time and st_*time_ns fields.> I fixed this in trunk last September> (issue 12904); os.utime now preserves all the precision that Python> currently conveys.Let's try in a ext4 filesystem:$ ~/prog/python/timestamp/pythonPython 3.3.0a0 (default:35d6cc531800+, Feb 16 2012, 13:32:56)>>> import decimal, os, shutil, time>>> open("test", "x").close()>>> shutil.copy2("test", "test2")>>> os.stat("test", timestamp=decimal.Decimal).st_mtimeDecimal('1329395871.874886224')>>> os.stat("test2", timestamp=decimal.Decimal).st_mtimeDecimal('1329395871.873350282')>>> os.stat("test2", timestamp=decimal.Decimal).st_mtime - os.stat("test", timestamp=decimal.Decimal).st_mtimeDecimal('-0.001535942')So shutil.copy2() failed to copy the timestamp: test2 is 1 ms older than test...Let's try with a program not written in Python: GNU make. The makefile:---------test2: test @echo "Copy test into test2" @~/prog/python/default/python -c 'import shutil;shutil.copy2("test", "test2")'test: @echo "Create test" @touch testclean: rm -f test test2---------First try:$ make cleanrm -f test test2$ makeCreate testCopy test into test2$ makeCopy test into test2=> test2 is always older than test and so is always "regenerated".Second try:$ make cleanrm -f test test2$ makeCreate testCopy test into test2$ makemake: `test2' is up to date.=> oh, here test2 is newer or has the exact same modification time, sothere is no need to rebuild it.Victor
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