1、if 的作用在于为代码创建了一个所谓的“分支”。if 语句告诉脚本：如果这个布尔表达式为真，就运行接下来的代码。
2、行尾的冒号作用在于告诉python 接下来要创建一个新的代码块，这和创建函数是一个道理。
3、在pyhton 的运行规则里，只要一行一（：）结尾，接下来的内容就应该有缩进。
4、如果python遇到多个elif块都是True,那么只有第一个为True的块会运行。
下边是ex29的代码。
1 #-*- coding: UTF-8 -*- 2 people = 20 3 cats = 30 4 dogs = 15 5 6 7 8 9 if people < cats : 10 print "Too many cats ,the world is doomed!" 11 12 if people > cats: 13 print "Not many cats , the world is saved." 14 15 if people < dogs: 16 print "The world is drooled on!" 17 18 if people > dogs: 19 print " The world is dry." 20 21 22 dogs +=5 23 24 if people >= dogs : 25 print"people are greater than or equal to dogs." 26 27 if people <= dogs : 28 print "people are less than or equal to dogs." 29 30 if people == dogs: 31 print "people are dogs." 32 33 #在if 语句中加入布尔值 34 if False or True: 35 print "people are dogs." 36 37 if False and True: 38 print "people are dogs."
下面是ex30的代码：
1 #-*- coding: UTF-8 -*- 2 people = 30 3 cars = 40 4 buses = 15 5 6 if cars > people: 7 print "we should take cars." 8 elif cars < people: 9 print "We should not take cars." 10 else: 11 print "We can't decide." 12 13 if buses > cars: 14 print "That's too many buses." 15 elif buses < cars: 16 print "maybe we should take buses." 17 else: 18 print "We still can't decide." 19 20 21 if people > buses: 22 print"Alright,let's just take the buses." 23 else: 24 print "Fine,let's stay home then." 25 26 #稍微复杂一点的布尔表达式 27 if people > cars and cars <buses: 28 print "YOhooooo" 29 else: 30 print "Fine,let's stop here."