[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 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").stmtime == os.stat("bar").stmtime

It works because Python uses float for utime() and for stat(). But this assertion may fail if another program checks file timestamps without lossing precision (because of float), e.g. a program written in 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/python Python 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).stmtime Decimal('1329395871.874886224') os.stat("test2", timestamp=decimal.Decimal).stmtime Decimal('1329395871.873350282') os.stat("test2", timestamp=decimal.Decimal).stmtime - os.stat("test", timestamp=decimal.Decimal).stmtime Decimal('-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 test clean: rm -f test test2

First try:

$ make clean rm -f test test2 $ make Create test Copy test into test2 $ make Copy test into test2

=> test2 is always older than test and so is always "regenerated".

Second try:

$ make clean rm -f test test2 $ make Create test Copy test into test2 $ make make: `test2' is up to date.

=> oh, here test2 is newer or has the exact same modification time, so there is no need to rebuild it.

Victor

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