Why does not else: run when if is false?

Question

var1 = float(input("Variable 1:"))
operator = input("Write your operator:")
if operator == "+" or "/" or "*" or "-" or "**":
    var2 = float(input("Variable 2:"))
    if operator == "+":  # Addition
        print(f"{var1} + {var2} = ")
        print(var1+var2)
    elif operator == "*":  # Multiplication
        print(f"{var1} * {var2} = ")
        print(var1 * var2)
    elif operator == "/":  # Division
        print(f"{var1} / {var2} = ")
        print(var1/var2)
    elif operator == "-":  # Subtraction
        print(f"{var1} - {var2} = ")
        print(var1 - var2)
    elif operator == "**":  # Power
        print(f"{var1} ** {var2} = ")
        print(var1 ** var2)
else:
    print("Invalid operation, enter a correct operator")

I have just started to programme in Python and I have decided to create a calculator that does basic mathematical operations. It is supossed to work correctly when I put the correct symbol into the "operator" variable, but when I put some random letter it does not run the command "else". Why does this happen?

Python is not English. `or` does not mean what you think it means. — Peter Wood, Jun 25 '22 at 11:04
You want to check whether `operator` is in a `set`: `if operator in {"+", "/", "*", "-", "**"}` — Peter Wood, Jun 25 '22 at 11:05
Does this answer your question? [Python's equivalent of && (logical-and) in an if-statement](https://stackoverflow.com/questions/2485466/pythons-equivalent-of-logical-and-in-an-if-statement) — Christoph Rackwitz, Jun 27 '22 at 08:00

score 3 · Accepted Answer · answered Jun 25 '22 at 11:15

3

You're applying the wrong logic in the code

operator == "+" or "/" or "*" or "-" or "**" is just comparing operator and + symbol, Ignoring rest operators

Correct way is

if operator == "+" or operator == "-" or operator == "*" or operator == "/" or operator = "**

I'll implement this like

if operator in ("+", "-", "*", "/", "**")

answered Jun 25 '22 at 11:15

Sidharth Mudgil

1,293
8
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No, the expression is ignoring the result of `operator == "+"` because `"/"` always evaluates `True`. – Peter Wood Jun 27 '22 at 14:55

score 1 · Answer 2 · answered Jun 25 '22 at 11:05

1

You can use in operator in this case to check if the operator is correctly written by the user:

if operator in {"+", "/", "*", "-", "**"}:

answered Jun 25 '22 at 11:05

Cardstdani

4,999
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score 0 · Answer 3 · edited Jun 25 '22 at 11:28

0

You need to use () for better understanding man. you can get your concept by using like this

if operator == ("+" or "/" or "*" or "-" or "**"):

edited Jun 25 '22 at 11:28

Mehrdad Pedramfar

10,941
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answered Jun 25 '22 at 11:25

ecemsm

1
1

score 0 · Answer 4 · answered Jun 25 '22 at 11:36

0

operator == "+" or "/"

will always evaluate to True in the if expression, because the second operand "/" will be implicitly casted to True (as any other non-empty string).

answered Jun 25 '22 at 11:36

klimenkov

347
2
8

Why does not else: run when if is false?

4 Answers4