[Python-Dev] Store timestamps as decimal.Decimal objects

[Victor Stinner]

Hi,

In issues #13882 and #11457, I propose to add an argument to functions
returning timestamps to choose the timestamp format. Python uses float
in most cases whereas float is not enough to store a timestamp with a
resolution of 1 nanosecond. I added recently time.clock_gettime() to
Python 3.3 which has a resolution of a nanosecond. The (first?) new
timestamp format will be decimal.Decimal because it is able to store
any timestamp in any resolution without loosing bits. Instead of
adding a boolean argument, I would prefer to support more formats. My
last patch provides the following formats:

 - "float": float (used by default)
 - "decimal": decimal.Decimal
 - "datetime": datetime.datetime
 - "timespec": (sec, nsec) tuple # I don't think that we need it, it
is just another example

The proposed API is:

  time.time(format="datetime")
  time.clock_gettime(time.CLOCK_REALTIME, format="decimal")
  os.stat(path, timestamp="datetime)
  etc.

This API has an issue: importing the datetime or decimal object is
implicit, I don't know if it is really an issue. (In my last patch,
the import is done too late, but it can be fixed, it is not really a
matter.)

Alexander Belopolsky proposed to use
time.time(format=datetime.datetime) instead.

--

The first step would be to add an argument to functions returning
timestamps. The second step is to accept these new formats (Decimal?)
as input, for datetime.datetime.fromtimestamp() and os.utime() for
example.

(Using decimal.Decimal, we may remove os.utimens() and use the right
function depending on the timestamp resolution.)

--

I prefer Decimal over a dummy tuple like (sec, nsec) because you can
do arithmetic on it: t2-t1, a+b, t/k, etc. It stores also the
resolution of the clock: time.time() and time.clock_gettime() have for
example different resolution (sec, ms, us for time.time() and ns for
clock_gettime()).

The decimal module is still implemented in Python, but there is
working implementation in C which is much faster. Store timestamps as
Decimal can be a motivation to integrate the C implementation :-)

--

Examples with the time module:

$ ./python
Python 3.3.0a0 (default:52f68c95e025+, Jan 26 2012, 21:54:31)
>>> import time
>>> time.time()
1327611705.948446
>>> time.time('decimal')
Decimal('1327611708.988419')
>>> t1=time.time('decimal'); t2=time.time('decimal'); t2-t1
Decimal('0.000550')
>>> t1=time.time('float'); t2=time.time('float'); t2-t1
5.9604644775390625e-06
>>> time.clock_gettime(time.CLOCK_MONOTONIC, 'decimal')
Decimal('1211833.389740312')
>>> time.clock_getres(time.CLOCK_MONOTONIC, 'decimal')
Decimal('1E-9')
>>> time.clock()
0.12
>>> time.clock('decimal')
Decimal('0.120000')

Examples with os.stat:

$ ./python
Python 3.3.0a0 (default:2914ce82bf89+, Jan 30 2012, 23:07:24)
>>> import os
>>> s=os.stat("setup.py", timestamp="datetime")
>>> s.st_mtime - s.st_ctime
datetime.timedelta(0)
>>> print(s.st_atime - s.st_ctime)
52 days, 1:44:06.191293
>>> os.stat("setup.py", timestamp="timespec").st_ctime
(1323458640, 702327236)
>>> os.stat("setup.py", timestamp="decimal").st_ctime
Decimal('1323458640.702327236')

Victor