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

[Victor Stinner]

> 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

The loss of precision is not constant: it depends on the timestamp value.

Another example using the stat program:
------------
import decimal, os, shutil, time
try:
    os.unlink("test")
except OSError:
    pass
try:
    os.unlink("test2")
except OSError:
    pass
open("test", "x").close()
shutil.copy2("test", "test2")
print(os.stat("test", timestamp=decimal.Decimal).st_mtime)
print(os.stat("test2", timestamp=decimal.Decimal).st_mtime)
print(os.stat("test2", timestamp=decimal.Decimal).st_mtime -
os.stat("test", timestamp=decimal.Decimal).st_mtime)
os.system("stat test|grep ^Mod")
os.system("stat test2|grep ^Mod")
------------

Outputs:
------------
$ ./python x.py
1329398229.918858600
1329398229.918208829
-0.000649771
Modify: 2012-02-16 14:17:09.918858600 +0100
Modify: 2012-02-16 14:17:09.918208829 +0100

$ ./python x.py
1329398230.862858588
1329398230.861343658
-0.001514930
Modify: 2012-02-16 14:17:10.862858588 +0100
Modify: 2012-02-16 14:17:10.861343658 +0100

$ ./python x.py
1329398232.450858570
1329398232.450067044
-0.000791526
Modify: 2012-02-16 14:17:12.450858570 +0100
Modify: 2012-02-16 14:17:12.450067044 +0100

$ ./python x.py
1329398233.090858561
1329398233.090853761
-0.000004800
Modify: 2012-02-16 14:17:13.090858561 +0100
Modify: 2012-02-16 14:17:13.090853761 +0100
------------
The loss of precision is between 1 ms and 4 us. Decimal timestamps
display exactly the same value than the stat program: I don't see any
bug in this example.

Victor

PS: Don't try os.utime(Decimal) with my patch, the conversion from
Decimal to _PyTime_t does still use float internally (I know this
issue, it should be fixed in my patch) and so loss precision ;-)