我使用 Python 制作了一个非常基本的脚本（我是新手），我不知道为什么它不工作，因为它应该工作

Question

The problem is in the line 19, where the if condition is.

So when you run the code it should ask you for a first number, then the mathematical operation (+,plus or -,minus) and finally for the second number.

It works perfectly when you are adding (plus), but when you are trying to subtract then it shows you the message "Invalid Operation", i've already tried using other logical operators but it just doesn't work D:

Pls explain me what's the problem, 'cause i can't see it.

minus = ["-","minus"]
plus = ["+", "plus"]

print("""
    ===========================
            CALCULATOR
    ===========================

    1      2      3      +
    4      5      6      -
    7      8      9

    0      Total:
    ===========================
    ===========================
    """)
n1 = int(input("First Number: "))
operation = input("+ or - ")
if operation not in (minus,plus):
    print("Invalid Operation")
else:

    n2 = int(input("Second Number: "))

    if operation in minus:
        total_minus = n1-n2
        print(f"""
    ===========================
            CALCULATOR
    ===========================

    1      2      3      +
    4      5      6      -
    7      8      9

    0      Total: {total_minus}
    ===========================
    ===========================
        """)
    elif operation in plus:
        total_plus = n1 + n2
        print(f"""
    ===========================
            CALCULATOR
    ===========================

    1      2      3      +
    4      5      6      -
    7      8      9

    0      Total: {total_plus}
    ===========================
    ===========================
        """)

Answer 1

The expression operation not in (minus,plus) is testing to see if operation is one of the lists minus or plus in the tuple (minus, plus) . Since it's a string, it will never be either of those values.

I'd recommend creating a combined list of valid operations.

valid_operations = minus + plus # concatenate valid operations

then test to see if the operation that the user input is in that list.

if operation not in valid_operations:
    print("Invalid Operation")
else:
    ...

This way it's easy to extend your calculator to multiplication, division, etc.

Answer 2

You are joining lists by forming a tuple:

if operation not in (minus, plus):

What you actually want to do is add the lists together:

if operation not in minus + plus:

Answer 3

operation not in (minus,plus) will always be true. operation is a string, and (minus,plus) is a tuple containing two lists.

If you want to test if the string operation is in the minus list or the plus list, you could instead use:

if operation not in (minus + plus):

Answer 4

For me, your code doesn't work with either of + or -! I don't know why it works for you with + as I think it shouldn't!

You are correct that the problem is your if statement. When you write:

if operation not in (minus,plus):

You are saying "if operation does not equal ["-", "minus"] or ["+", "plus"] "

In other words you are comparing the input of the user to a list of 2 strings!

You can instead write:

if operation not in minus and operation not in plus:

and your program will work just fine

Answer 5

Just change in if block of code

if operation in plus or operation in minus:
    your code
else:
    print("Invalid Operation")

You wrote:

if operation not in (minus,plus):
    your code

This will always be True, since operation will never be in both lists (plus and minus) so that statement is False, and since you wrote 'not in', not of False is True, that's the reason you always get "Invalid Operation" in your if block.