说明：  用Python写的计算器，带中文注释 为了后面迭代算出有括号的表达式，我们先写一个没有括号的表达式，比如说像这样一个表达式 '100.5+40*5/2-3*2*2/4+9',对于这样的表达式我们肯定是计算乘除，在计算加减，计算一个最小计算单元后，再将结果放回列表中不断循环，直到算出整个不带括号的表达式
(Calculator written in Python with Chinese notes In order to calculate the bracketed expression in the next iteration, we first write an expression without brackets, such as'100.5+40*5/2-3*2*2/4+9'. For such an expression, we must calculate multiplication and division. After calculating addition and subtraction, we calculate a minimum computing unit, and then put the result back into the list and continue to cycle until the whole expression without brackets is calculated.)