python - 基本计算器只会添加值

Question

这个问题已经有答案了:

Why does "a == x or y or z" always evaluate to True? How can I compare "a" to all of those? (8 个回答)

已关闭 4 年前。

我是Python新手，正在尝试制作一个基本计算器，它接受两个数字并执行用户指定的操作，但无论我输入什么操作，数字总是相加。

num1 = int(input("Enter a number. "))
print(" ")
num2 = int(input("Enter another number. "))
print(" ")
operation = input("Type Add, Subtract, Multiply, or Divide" )

if operation == "Add" or "add":
    add = num1 + num2
    print(add)
elif operation == "Subtract" or "subtract":
    sub = num1 - num2
    print(sub)
elif operation == "Multiply" or "multiply":
    mult = num1 * num2
    print(mult)
elif operation == "Divide" or "divide":
    div = num1 / num2
    print(div)
else:
    print("Enter a valid operation: ")

根据我所拥有的，如果 num1 = 10 且 num2 = 5 并且我输入“乘”，我得到的结果是 15，而不是 50。

Answer 1

if operation == "Add" or "add"

这是两个条件:

1. if 操作 == "Add" 它将执行您所期望的操作。
2. if "add" - 始终计算为 True，因为它是常量字符串。所以你也可以这样写:如果为真。您需要将其替换为:

if operation == "Add" or operation == "add"

无论如何，我想提出一些建议。

小写，然后仅检查一次

您应该小写输入字符串，而不是仔细检查“Add”或“add”:

num1 = int(input("Enter a number. "))
print(" ")
num2 = int(input("Enter another number. "))
print(" ")
operation = input("Type Add, Subtract, Multiply, or Divide" ).lower()  # Notice the `lower` here

if operation == "add":
    add = num1 + num2
    print(add)
elif operation == "subtract":
    sub = num1 - num2
    print(sub)
elif operation "multiply":
    mult = num1 * num2
    print(mult)
elif operation "divide":
    div = num1 / num2
    print(div)
else:
    print("Enter a valid operation: ")

使用内置 operator模块

operator 模块恰好包含您需要的方法。为了方便起见，您可以使用它(尚未测试，应该有效):

import operator
keywords = {"add": "add", "subtract": "sub", "multiply": "mul", "divide": "truediv"}

num1 = int(input("Enter a number. "))
print(" ")
num2 = int(input("Enter another number. "))
print(" ")
operation = input("Type Add, Subtract, Multiply, or Divide" )
op_func = keywords.get(operation.lower(), "add")  # defaults to add
print(getattr(operator, op_func)(num1, num2))