Python input validation?

Question

I have a simple input validation code asking the user for either ACT or SAT.

test=input("Are you taking SAT or ACT?..")
while test!=("SAT" or "ACT"):
 print("error")
 test=input("Are you taking SAT or ACT?..")

It seems to work correctly for "SAT" which is in front on line 2, but not ACT! When I type in ACT in the module it will print "error" like it was false. What is my flaw here? Logic? Syntax? Semantic? What can I do to fix it?

Thank you.

Answer 1

I don't think that this statement is valid:

test!=("SAT" or "ACT")

You may use

test != "SAT" and test != "ACT"

Or use in operator:

test=input("Are you taking SAT or ACT?..")
while test not in ("SAT", "ACT"):
 print("error")
 test=input("Are you taking SAT or ACT?..")

• Michael Jaros: The statement is perfectly valid, but it does not yield the expected results. Operators in programming languages work in a very simple, well-defined way. On the other hand this means, that they cannot be used like in conversation language. Compare for example this pseudocode "if test is not a or b" (conversational) to "if (test is not a) and (test is not b)" (strict).
• Ammar Alyousfi: Yes. You are right. I meant by "not valid" that the statement doesn't mean anything useful here, because it doesn't achieve what OP wanted, and because "SAT" or "ACT" always yield the same thing, "SAT".
• Pup Watthanawong: thank you, I get it now-- no matter ACT or SAT, the while loop will always be true when I use "or".
• Ammar Alyousfi: @PupWatthanawong In fact, when you enter "SAT" the condition of the while loop will evaluate to False, because "SAT" or "ACT" always evaluates to "SAT".

Answer 2

The expression ("SAT" or "ACT") evaluates to "SAT" since it basically evaluates the OR operation on two strings. In order to fix the issue, this is what you can do:

while(1):
 test = input("Are you taking SAT or ACT")
 if test in ("SAT", "ACT"):
 break
 print("Error")

• Yann Vernier: Using the not in operator would make for a smaller change. Also, no need for parenthesis in a while True:.

Answer 3

Teverse the condition from test!=("SAT" or "ACT") to test==("SAT" or "ACT") or you can use IN as explained by Kshitij Saraogi.