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

[Antoine Pitrou]

On Thu, 16 Feb 2012 13:46:18 +0100
Victor Stinner <victor.stinner at gmail.com> wrote:
> 
> 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).st_mtime
> Decimal('1329395871.874886224')
> >>> os.stat("test2", timestamp=decimal.Decimal).st_mtime
> Decimal('1329395871.873350282')

This looks fishy. Floating-point numbers are precise enough to
represent the difference between these two numbers:

>>> f = 1329395871.874886224
>>> f.hex()
'0x1.3cf3e27f7fe23p+30'
>>> g = 1329395871.873350282
>>> g.hex()
'0x1.3cf3e27f7e4f9p+30'

If I run your snippet and inspect modification times using `stat`, the
difference is much smaller (around 10 ns, not 1 ms):

$ 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

In other words, you should check your PEP implementation for bugs.

Regards

Antoine.