What is wrong with this code? Python 'while not' script

Question

I want to have the program check if the code is or is not 'w', 'W', 'd' or 'D' and then direct the user from there, but as it is, it always goes to invalid() even when the input is 'w', 'W', 'd' or 'D'. And I don't want to use 'while code is equal to 'w', 'W', 'd' or 'D' continue on' because even though that works while the program is running, it causes the program to start over from the beginning after it's finished.

Plus, I'm not allowed to do the simple way using if statements

if code == 'W' or code == 'w' or code == 'D' or code == 'd':
    prevbalance = float(input('Enter your previous balance: $'))
else:
    invalid()

Here is the problem area:

code = input('Enter your transaction code: ')
while not code == 'W' or code == 'w' or code == 'D' or code == 'd':
    invalid()
prevbalance = float(input('Enter your previous balance: $'))

Here is the entire program if it helps:

def invalid():
    input('Invalid transaction code. Please select another code.')
    main()

def deposit(prevbalance, amount, code):
    Wbalance = ''
    Dbalance = prevbalance + amount
    balance(Wbalance, Dbalance, code)

def withdrawal(prevbalance, amount, code):
    Dbalance = ''
    Wbalance = prevbalance - amount
    if amount > prevbalance:
        print('~~ERROR~~  You cannot withdrawal more than you have')
        input('           Please try a lower amount back at the main menu')
        main()
    else:
        balance(Wbalance, Dbalance, code)

def balance(Wbalance, Dbalance, code):
    if code == "W" or code == "w":
        print('Your new balance is: $',format(Wbalance, ',.2f'),)
    else:
        print('Your new balance is: $',format(Dbalance, ',.2f'),)

def main():
    name = input('Enter your name: ')
    ID = input('Enter your account ID: ')
    code = input('Enter your transaction code: ')
    while not code == 'W' or code == 'w' or code == 'D' or code == 'd':
        invalid()
    prevbalance = float(input('Enter your previous balance: $'))
    amount = float(input('Enter your transaction amount: $'))
    if code == "W" or code == "w":
        withdrawal(prevbalance, amount, code)
    elif code == "D" or code == "d":
            deposit(prevbalance, amount, code)

main()
input('Press ENTER to continue...')

`Invalid` function should return control to the calling point, and not call `main()`. — Thane Plummer, Oct 19 '15 at 17:53
Check out the solution in this thread. http://stackoverflow.com/questions/21122540/python-input-error-nameerror-name-is-not-defined The `input` keyword may not be doing what you think — ate50eggs, Oct 19 '15 at 17:56

tzaman · Answer 1 · 2015-10-19T21:14:21.393

3

not doesn't distribute across all of your conditions. You need to add parentheses:

while not (code == 'W' or code == 'w' or code == 'D' or code == 'd'):

Or better yet, use the in operator:

while code not in ('W', 'w', 'D', 'd'):

edited Oct 19 '15 at 21:14

answered Oct 19 '15 at 17:54

tzaman

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`while code not in 'WwDd':` will also accept `wD` ? – Muhammad Tahir Oct 19 '15 at 18:01

score 2 · Answer 2 · answered Oct 19 '15 at 17:54

2

In your while condition not will negate only code=='W' not other conditions.

So you would probably need to do:

while code not in ('W', 'w', 'D', 'd'):

answered Oct 19 '15 at 17:54

Muhammad Tahir

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score 2 · Answer 3 · answered Oct 19 '15 at 17:58

This is an operator precedence problem. In the following:

while not code == 'W' or code == 'w' or code == 'D' or code == 'd':

the "not" operator binds more tightly than "or", so it is equivalent to:

while (not code == 'W') or code == 'w' or code == 'D' or code == 'd':

which is not what was intended. You can fix it with parentheses:

while not (code == 'W' or code == 'w' or code == 'D' or code == 'd'):

Or by using De Morgan's laws:

while code != 'W' and code != 'w' and code != 'D' and code != 'd':

But I suggest the following:

while code not in ('W', 'w', 'D', 'd'):

Hope that helps.

Thank you so much!! I was so confused as to why it wasn't working. and I have tried '!=', however it wouldn't work either, though i could have been putting it in wrong. Either way, thanks again for the help! — Matthew Jesus Believer Tompkin, Oct 19 '15 at 19:36

What is wrong with this code? Python 'while not' script

3 Answers3