<a href="http://www.jianshu.com/p/54870e9541fc">总目录</a>
一、问题
Python 2 默认的/是integer division, 所以在计算结果需要floats的时候，要做一下处理
- 被除数如果知道是多少，比如88，就写成
88.0或者88.。 - 被除数如果是变量，就要乘以
1.0。
看起来很简单的问题，但是在实际操作中很容易就弄错了。
比如在speed_fraction这个练习中:
# -*- coding: utf-8 -*-
# Write a procedure, speed_fraction, which takes as its inputs the result of
# a traceroute (in ms) and distance (in km) between two points. It should
# return the speed the data travels as a decimal fraction of the speed of
# light.
speed_of_light = 300000. # km per second
def speed_fraction(traceroute, distance):
speed = distance / (traceroute*1.0/2/1000.)
print "Correct traceroute*1.0/2/1000.", traceroute/2/1000.
print "Correct speed", speed
return speed / (speed_of_light)
def speed_fraction_wrong(traceroute, distance):
speed = distance / (traceroute/2/1000.)
print "WRONG traceroute/2/1000.", traceroute/2/1000.
print "WRONG speed", speed
result = speed / speed_of_light
return result
def speed_fraction_1(traceroute, distance):
speed = distance / (traceroute/2000.)
print "Correct traceroute/2000.", traceroute/2/1000.
print "Correct speed", speed
result = speed / speed_of_light
return result
print speed_fraction(50,5000), "\n\n"
#>>> 0.666666666667
print speed_fraction(50,10000), "\n\n"
#>>> 1.33333333333 # Any thoughts about this answer, or these inputs?
print speed_fraction(75,4500), "\n\n"
#>>> 0.4
print "WRONG %s" % speed_fraction_wrong(50,5000), "\n\n"
#>>> 0.666666666667
print "WRONG %s" % speed_fraction_wrong(50,10000), "\n\n"
#>>> 1.33333333333 # Any thoughts about this answer, or these inputs?
print "WRONG %s" % speed_fraction_wrong(75,4500), "\n\n"
#>>> 0.4
print "speed_fraction_1 %s" % speed_fraction_1(75,4500), "\n\n"
#>>> 0.4
Console:
Correct traceroute*1.0/2/1000. 0.025
Correct speed 200000.0
0.666666666667
Correct traceroute*1.0/2/1000. 0.025
Correct speed 400000.0
1.33333333333
Correct traceroute*1.0/2/1000. 0.037
Correct speed 120000.0
0.4
WRONG traceroute/2/1000. 0.025
WRONG speed 200000.0
WRONG 0.666666666667
WRONG traceroute/2/1000. 0.025
WRONG speed 400000.0
WRONG 1.33333333333
WRONG traceroute/2/1000. 0.037
WRONG speed 121621.621622
WRONG 0.405405405405
Correct traceroute/2000. 0.037
Correct speed 120000.0
speed_fraction_1 0.4
Process finished with exit code 0
二、症结
问题的症结所在：
-
speed = distance / (traceroute*1.0/2/1000.)对； -
speed = distance / (traceroute/2000.)对 -
speed = distance / (traceroute/2/1000.)错； - 即使
(traceroute*1.0/2/1000.)和(traceroute/2000.)和(traceroute/2/1000.)结果都是0.037
很有意思，不过python2这点也真的很搞... 总之见到被除数就乘以1.0好了，万一出问题debug起来可是很伤神啊。
三、参考
- 这个网站很好地比较了Python3 和 Python2，如果上述代码在Python3中运行，就不会出现问题了。
