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.

Ammar Alyousfi · Accepted Answer · 2017-11-15 06:27:24Z

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?..")

answered Nov 15, 2017 at 6:27

Ammar Alyousfi

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4 Comments

Michael Jaros Over a year ago

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 Over a year ago

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 Over a year ago

thank you, I get it now-- no matter ACT or SAT, the while loop will always be true when I use "or".

Ammar Alyousfi Over a year ago

@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".

Kshitij Saraogi · Accepted Answer · 2017-11-15 06:29:58Z

1

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")

answered Nov 15, 2017 at 6:29

Kshitij Saraogi

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1 Comment

Yann Vernier Over a year ago

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

Neil · Accepted Answer · 2017-11-15 07:56:49Z

0

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

edited Nov 15, 2017 at 7:56

Neil

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answered Nov 15, 2017 at 7:51

iways

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