变量的作用：
Variables are uesd to store information to be referenced and manipulated in a computer . they also provide a way of labeling data with a descriptive name, so our programs can be understood more clearly by the reader and ourselves. it is helpful to think of variablese as containers that hold information their sole purpose is to label and store data in memory this data can then be uesd throughout your program
声明变量：
name = "alex"
name为变量名
alex为变量值
定义变量的规则：
变量名只能是 字母，数字 下划线的任意组合；
变量名的第一个字符不能是数字；
python中的语法不能作为变量命，如：print an in 等等
约定俗成的固定：
变量名不能用中文；
变量名不能用拼英；
变量名不能过长；
变量名不能词不达意
常量：
python中没有常量的固定语法，和变量形式一样，约定俗成将变量名全部大写视为常量。
