Question

I recently used the timeit module to do a very simple performance test of the python. The result really stunned me: the time consumed by x=x+x is about 125 times of x+x or y=x+x, why?! I really hope someone will give me some clue about this, maybe I used the timeit wrong? Thanks!

Please notice that y=x+x;x=y is as slow as x=x+x… but the x=x+47 is as fast as x+x

testBasicOps()

testcase=”pass”, time lapse:0.001487secs

testcase=”x=47″, time lapse:0.002424secs

testcase=”x=94″, time lapse:0.002423secs

testcase=”x=47*2″, time lapse:0.002423secs

testcase=”x+x”, time lapse:0.003922secs

testcase=”x*2″, time lapse:0.005307secs

testcase=”x=x+x”, time lapse:0.497974secs

testcase=”x=x*2″, time lapse:0.727506secs

testcase=”x=x+47″, time lapse:0.005770secs

testcase=”x=47+x”, time lapse:0.004442secs

testcase=”x+=x”, time lapse:0.498920secs

testcase=”y=x+x”, time lapse:0.004102secs

testcase=”y=x*2″, time lapse:0.006327secs

testcase=”y=x+x

x=y”, time lapse:0.499644secs

testcase=”x+x

y=x”, time lapse:0.004948secs

testcase=”x+x

x=y”, time lapse:0.005126secs

testcase=”y=10

x=y”, time lapse:0.003351secs

testcase=”pass”, time lapse:0.001487secs

The code I used:

import timeit
import numpy as npy
def testBasicOps():
    timeitSetup="""
x=47
y=0
"""
    testCases=['pass','x=47',\
               'x=94','x=47*2'\
               ,'x+x','x*2'\
               ,'x=x+x','x=x*2'\
               ,'x=x+47','x=47+x'\
               ,'x+=x','y=x+x'\
               ,'y=x*2','y=x+x\nx=y'\
               ,'x+x\ny=x','x+x\nx=y'\
               ,'y=10\nx=y']
    minT=[]
    tests=[]
    for i in testCases:
        tests.append(timeit.Timer(i,setup=timeitSetup))
        minT.append(npy.mean(tests[-1].repeat(10,int(1e5))))
        print 'testcase=\"%s\", time lapse:%fsecs'%(i,minT[-1])

def main():
    print "#"*10
    print "testBasicOps()"
    testBasicOps()

if __name__ == '__main__':
    main()        

Answer 1

When you compute x=x+x many thousands of times, x becomes a very large. You’re measuring the length of time it takes to add two very large numbers.