1、基本构成：
if 条件：
if语句块
else:
else语句
2、用于比较运算（大于>）中：
a = 0
if a > 0:
print "a is not 0"
else:
print 'a is o'
3、用于比较运算中结合逻辑运算符：
a = 50
if a< 100 and a > 10:
print "a is not 0"
else:
print 'a is false'
4、and的优先级大于or有括号的运算最优先：
a = 50
if (a< 100 and a > 10 or (a >20 and a<100):
print "a is true"
else:
print 'a is false'
5、结合比较运算操作符: >< == >= <= == != <>：
a =90
b =100
if a>b:
print "a is max"
else:
print 'a is min'
6、结合不等于实例：
a =90
b =100
if a<>b:
print "a is max"
else:
print 'a is min'
7、结合成员关系运算符：In （not in ）：
name = 'zhangshan'
if 'zhang' not in name:
print 'zhang is in name'
else:
print 'zhang is not in name'
8、elif嵌套结构：
if 条件：
if语句块
elif 条件：
elif语句块
else:
else语句块
9、用于检查多个条件是否满足：
number1 = int(input("请输入数字1:"))
number2 = int(input("请输入数字2:"))
if number1 > number2:
print "{} 大于 {}".format(number1,number2)
elif number2 < number2:
print "{} 小于 {}".format(number1,number2)
elif number1 == number2:
print '%s 等于 %s'%(number1,number2)
else:
print 'game is over'
10、嵌套语句2：
name = input("请输入信息:")
if name.endswith('hello'):
if name.startswith('china'):
print 'welcome to {}'.format(name)
elif name.startswith('japan'):
print 'say you {}'.format(name)
else:
print '输入有误，重新输入'
else:
print '游戏结束---->'