High-precision clock in Python

Question

Is there a way to measure time with high-precision in Python --- more precise than one second? I doubt that there is a cross-platform way of doing that; I'm interesting in high precision time on Unix, particularly Solaris running on a Sun SPARC machine.

timeit seems to be capable of high-precision time measurement, but rather than measure how long a code snippet takes, I'd like to directly access the time values.

Answer 1

Python tries hard to use the most precise time function for your platform to implement time.time():

/* Implement floattime() for various platforms */

static double
floattime(void)
{
    /* There are three ways to get the time:
      (1) gettimeofday() -- resolution in microseconds
      (2) ftime() -- resolution in milliseconds
      (3) time() -- resolution in seconds
      In all cases the return value is a float in seconds.
      Since on some systems (e.g. SCO ODT 3.0) gettimeofday() may
      fail, so we fall back on ftime() or time().
      Note: clock resolution does not imply clock accuracy! */
#ifdef HAVE_GETTIMEOFDAY
    {
        struct timeval t;
#ifdef GETTIMEOFDAY_NO_TZ
        if (gettimeofday(&t) == 0)
            return (double)t.tv_sec + t.tv_usec*0.000001;
#else /* !GETTIMEOFDAY_NO_TZ */
        if (gettimeofday(&t, (struct timezone *)NULL) == 0)
            return (double)t.tv_sec + t.tv_usec*0.000001;
#endif /* !GETTIMEOFDAY_NO_TZ */
    }

#endif /* !HAVE_GETTIMEOFDAY */
    {
#if defined(HAVE_FTIME)
        struct timeb t;
        ftime(&t);
        return (double)t.time + (double)t.millitm * (double)0.001;
#else /* !HAVE_FTIME */
        time_t secs;
        time(&secs);
        return (double)secs;
#endif /* !HAVE_FTIME */
    }
}

( from http://svn.python.org/view/python/trunk/Modules/timemodule.c?revision=81756&view=markup )

Answer 2

I observed that the resolution of time.time() is different between Windows 10 Professional and Education versions.

On a Windows 10 Professional machine, the resolution is 1 ms. On a Windows 10 Education machine, the resolution is 16 ms.

Fortunately, there's a tool that increases Python's time resolution in Windows: https://vvvv.org/contribution/windows-system-timer-tool

With this tool, I was able to achieve 1 ms resolution regardless of Windows version. You will need to be keep it running while executing your Python codes.

Answer 3

David's post was attempting to show what the clock resolution is on Windows. I was confused by his output, so I wrote some code that shows that time.time() on my Windows 8 x64 laptop has a resolution of 1 msec:

# measure the smallest time delta by spinning until the time changes
def measure():
    t0 = time.time()
    t1 = t0
    while t1 == t0:
        t1 = time.time()
    return (t0, t1, t1-t0)

samples = [measure() for i in range(10)]

for s in samples:
    print s

Which outputs:

(1390455900.085, 1390455900.086, 0.0009999275207519531)
(1390455900.086, 1390455900.087, 0.0009999275207519531)
(1390455900.087, 1390455900.088, 0.0010001659393310547)
(1390455900.088, 1390455900.089, 0.0009999275207519531)
(1390455900.089, 1390455900.09, 0.0009999275207519531)
(1390455900.09, 1390455900.091, 0.0010001659393310547)
(1390455900.091, 1390455900.092, 0.0009999275207519531)
(1390455900.092, 1390455900.093, 0.0009999275207519531)
(1390455900.093, 1390455900.094, 0.0010001659393310547)
(1390455900.094, 1390455900.095, 0.0009999275207519531)

And a way to do a 1000 sample average of the delta:

reduce( lambda a,b:a+b, [measure()[2] for i in range(1000)], 0.0) / 1000.0

Which output on two consecutive runs:

0.001
0.0010009999275207519

So time.time() on my Windows 8 x64 has a resolution of 1 msec.

A similar run on time.clock() returns a resolution of 0.4 microseconds:

def measure_clock():
    t0 = time.clock()
    t1 = time.clock()
    while t1 == t0:
        t1 = time.clock()
    return (t0, t1, t1-t0)

reduce( lambda a,b:a+b, [measure_clock()[2] for i in range(1000000)] )/1000000.0

Returns:

4.3571334791658954e-07

Which is ~0.4e-06

An interesting thing about time.clock() is that it returns the time since the method was first called, so if you wanted microsecond resolution wall time you could do something like this:

class HighPrecisionWallTime():
    def __init__(self,):
        self._wall_time_0 = time.time()
        self._clock_0 = time.clock()

    def sample(self,):
        dc = time.clock()-self._clock_0
        return self._wall_time_0 + dc

(which would probably drift after a while, but you could correct this occasionally, for example dc > 3600 would correct it every hour)

Answer 4

You can also use time.clock() It counts the time used by the process on Unix and time since the first call to it on Windows. It's more precise than time.time().

It's the usually used function to measure performance.

Just call

import time
t_ = time.clock()
#Your code here
print 'Time in function', time.clock() - t_

EDITED: Ups, I miss the question as you want to know exactly the time, not the time spent...

Answer 5

The comment left by tiho on Mar 27 '14 at 17:21 deserves to be its own answer:

In order to avoid platform-specific code, use timeit.default_timer()

Answer 6

Python 3.7 introduces 6 new time functions with nanosecond resolution, for example instead of time.time() you can use time.time_ns():

import time
print(time.time())
# 1522915698.3436284
print(time.time_ns())
# 1522915698343660458

These 6 functions are described in PEP 564:

time.clock_gettime_ns(clock_id)

time.clock_settime_ns(clock_id, time:int)

time.monotonic_ns()

time.perf_counter_ns()

time.process_time_ns()

time.time_ns()

These functions are similar to the version without the _ns suffix, but return a number of nanoseconds as a Python int.