Variable to store some intermediate results of the program during the operation, in order to facilitate future calls.
Variable naming rules:
1, be descriptive
2, the variable name only _, numbers, letters, spaces or special characters are not (#? <., $ *! ~)
3, Chinese can not be a variable name (not recommended, rules)
4, can not start with a number
5, reserved characters can not be used
Constants : a constant amount, such as pi pie = 3.141592653 ...
In python which all variables are variable, so use all uppercase variable names to represent this variable is constant.
