python 根据运算符实现四则运算

方法一，先来一个自己写的简单版 

def arithmetic(a, b, operator):
    if operator == "add":
        return a + b
    elif operator == "subtract":
        return a - b
    elif operator == "multiply":
        return a * b
    elif operator == "divide":
        return a / b

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方法二，利用字典实现四则运算，然后根据operator给定的字符实现四则运算的选择

def arithmetic(a, b, operator): 

    return { 'add': a + b, 'subtract': a - b, 'multiply': a * b, 'divide': a / b, }[operator]