Software timing benchmarks are notoriously noisy because your computer isn't a very good controlled experimental platform.

• Some algorithms are nondeterministic. For instance, some quicksort implementations choose a random pivot element.
• While you're running your benchmarks, your python process may get preempted by other running applications, operating system threads, etc.
• Even if your process got the same amount of CPU during every run, the timing may be different because a different mix of applications is running so your processor's memory caches have different hit/miss patterns.
• Some runs may get better branch prediction than others (again, due to the mix of other processes running). This will cause more pipeline stalls.

I would suspect that the effects I've given are given in order of decreasing strength (except that I believe Python's sort algorithm is deterministic, so the first effect is not a factor here). I also suspect that I've just scratched the surface.

Software timing benchmarks are notoriously noisy because your computer isn't a very good controlled experimental platform.

• Some algorithms are nondeterministic. For instance, some quicksort implementations choose a random pivot element.
• While you're running your benchmarks, your python process may get preempted by other running applications, operating system threads, etc.
• Even if your process got the same amount of CPU during every run, the timing may be different because a different mix of applications is running so your processor's memory caches have different hit/miss patterns.
• Some runs may get better branch prediction than others (again, due to the mix of other processes running). This will cause more pipeline stalls.

I would suspect that the effects I've given are given in order of decreasing strength (except that I believe Python's sort algorithm is deterministic, so the first effect is not a factor here). I also suspect that I've just scratched the surface.

EDIT:

OK, for reversing a number, suppose that your typical input will be 5 digits then you can use this.

if __name__ == "__main__":
    from timeit import Timer
    import random
    def test(f):
        f(random.randint(10000,99999))

    print Timer(lambda: test(reverse_num)).timeit(number = 10000000)
    print Timer(lambda: test(reverse_num2)).timeit(number = 10000000)