Measuring elapsed time in python

What you seem to be looking for is a monotonic timer. A monotonic time reference does not jump or go backwards.

There have been several attempts to implement a cross platform monotomic clock for Python based on the OS reference of it. (Windows, POSIX and BSD are quite different) See the discussions and some of the attempts at monotonic time in this SO post.

Mostly, you can just use os.times():

os.times()

Return a 5-tuple of floating point numbers indicating accumulated (processor or other) times, in seconds. The items are: user time, system time, children’s user time, children’s system time, and elapsed real time since a fixed point in the past, in that order. See the Unix manual page times(2) or the corresponding Windows Platform API documentation. On Windows, only the first two items are filled, the others are zero.

Availability: Unix, Windows

But that does not fill in the needed elapsed real time (the fifth tuple) on Windows.

If you need Windows support, consider ctypes and you can call GetTickCount64() directly, as has been done in this recipe.

Python 3.3 added a monotonic timer into the standard library, which does exactly what I was looking for. Thanks to Paddy3118 for pointing this out in "How do I get monotonic time durations in python?".

For measuring elapsed CPU time, look at time.clock(). This is the equivalent of Linux's times() user time field.

For benchmarking, use timeit.

The datetime module, which is part of Python 2.3+, also has microsecond time if supported by the platform.

Example:

>>> import datetime as dt
>>> n1=dt.datetime.now()
>>> n2=dt.datetime.now()
>>> (n2-n1).microseconds
678521
>>> (n2.microsecond-n1.microsecond)/1e6
0.678521
ie, it took me .678521 seconds to type the second n2= line -- slow
>>> n1.resolution
datetime.timedelta(0, 0, 1)
1/1e6 resolution is claimed.

If you are concerned about system time changes (from DS -> ST) just check the object returned by datetime.Presumably, the system time could have a small adjustment from an NTP reference adjustment. This should be slewed, and corrections are applied gradually, but ntp sync beats can have an effect with very small (millisec or microsec) time references.

You can also reference Alex Martelli's C function if you want something of that resolution. I would not go too far to reinvent the wheel. Accurate time is basic and most modern OS's do a pretty good job.

Edit

Based on your clarifications, it sounds like you need a simple side check if the system's clock has changed. Just compare to a friendly, local ntp server:

import socket
import struct
import time

ntp="pool.ntp.org"   # or whatever ntp server you have handy

client = socket.socket( socket.AF_INET, socket.SOCK_DGRAM )
data = '\x1b' + 47 * '\0'
client.sendto( data, ( ntp, 123 ))
data, address = client.recvfrom( 1024 )
if data:
    print 'Response received from:', address
    t = struct.unpack( '!12I', data )[10]
    t -= 2208988800L #seconds since Epoch
    print '\tTime=%s' % time.ctime(t)

NTP is accurate to milliseconds over the Internet and has representation resolution of resolution of 2−32 seconds (233 picoseconds). Should be good enough?

Be aware that the NTP 64 bit data structure will overflow in 2036 and every 136 years thereafter -- if you really want a robust solution, better check for overflow...