Python 程序优化

有2段代码如下

a.py

#!/usr/bin/python

from time import time
t=time()

lista=[1,2,3,4,5,6,7,8,9,10]
listb=[0.1,0.2,0.3,0.4,0.5,0.6,0.7,0.8,0.9,0.01]

len1=len(lista)
len2=len(listb)

for  i in xrange(1000000):
      for a in xrange(len1):
         temp=lista[a]
         for b in xrange(len2):
            x=temp+listb[b]

print "total run time"
print time()-t



b.py

#!/usr/bin/python

from time import time
t=time()

lista=[1,2,3,4,5,6,7,8,9,10]
listb=[0.1,0.2,0.3,0.4,0.5,0.6,0.7,0.8,0.9,0.01]

len1=len(lista)
len2=len(listb)

for  i in xrange(1000000):
      for a in xrange(len1):
         for b in xrange(len2):
            x=lista[a]+listb[b]

print "total run time"
print time()-t

为什么执行的时间有下面的差别呢？

[root@cdn ~]# python a.py
total run time
39.4940960407
[root@cdn ~]# python b.py
total run time
43.6094748974