使用Python编写一个四则运算程序，要求加，减，乘，除各定义一个函数来实现

好的，以下是一个简单的四则运算程序的 Python 代码：

def add(a, b):
    return a + b

def subtract(a, b):
    return a - b

def multiply(a, b):
    return a * b

def divide(a, b):
    if b == 0:
        raise ValueError("Cannot divide by zero!")
    return a / b

operator = input("Enter an operator (+, -, *, /): ")
num1 = float(input("Enter the first number: "))
num2 = float(input("Enter the second number: "))

if operator == "+":
    print(num1, "+", num2, "=", add(num1, num2))
elif operator == "-":
    print(num1, "-", num2, "=", subtract(num1, num2))
elif operator == "*":
    print(num1, "*", num2, "=", multiply(num1, num2))
elif operator == "/":
    print(num1, "/", num2, "=", divide(num1, num2))
else:
    print("Invalid operator!")

你可以在命令行中运行这个程序，输入两个数字和一个运算符，程序会输出计算结果。