num += 1 等价于 num = num + 1
逻辑运算符
and 全true则true
条件1 and 条件2
5>3 and 3>2 ===> true
5>3 and 3<2 ===>false
or 有true则true
5>3 or 6<2 ===>true
not 取反
not 5>3 ===>false
not 5<3 ===>true
a>b and c>d or not f
判断优先级:最好加括号
not > and > or
原文：http://www.cnblogs.com/darkalex001/p/7487681.html