New code. How can I display an error message if the user entered invalid input?

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This is for my project. My problem is I don't know how to display an error message inside my class function code. Error message must display if the user entered a non-parenthesis character.

What I have tried:

open_list=['(','[','{']
close_list=[')',']','}']

def check(mystr):
    stack=[]
    for i in mystr:
        if i in open_list:
            stack.append(i)
        elif i in close_list:
            pos=close_list.index(i)
            if ((len(stack)>0)) and (open_list[pos]==stack[len(stack)-1]):
                stack.pop()
            else:
                return"Invalid"
            if len(stack)==0:
                return"Valid"
            else:
                return"Invalid"
                
while True:
        s=input("Enter your parentheses:")
        string=[{},[],()]
        print(s,'-',check(s))
        if s in string:
            print("Well done!")
        else:
            print("Error! please enter a valid parentheses")

I just need to put an error message if the user entered a non-parenthesis character.
Thank you in advance.

I need help in part of the error message. Error message must show if the user entered non-parentheses input.

Posted 11-Jan-21 20:59pm

lil_mint

Updated 13-Jan-21 20:05pm

v5

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Comments

Richard MacCutchan 12-Jan-21 6:34am

you need to iterate all the input characters and test to see that they are one of the valid set in list a. If you find a character that is not in the set then print the error message and reject the input string.

lil_mint 12-Jan-21 8:06am

I also had that in my mind but I don't know how to start or how to do it.. Do I need to make another def func? or ???..Also thanks

Richard MacCutchan 12-Jan-21 8:23am

Do it in function f. Something like:

for c in str:
    if c not in a:
        # a bad character

I am not sure why you have duplicated the valid characters in a, you should only need them once.

lil_mint 12-Jan-21 9:45am

Thank you for the ideas and for sharing what's wrong with it... I will try it. Thanks!

Richard MacCutchan 14-Jan-21 4:21am

You have updated your post, but it is not clear if it is now solved, or you have a further problem.

lil_mint 14-Jan-21 8:57am

it was still on the error message but now it was solved.. Thank you

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Sandeep Mewara · Accepted Answer · 2021-01-11T21:13:00

Would suggest you to use User-Defined Exception in Python using try-except

Refer: How to Define Custom Exceptions in Python? (With Examples)[^]

Example from there:

Python

# define Python user-defined exceptions
class Error(Exception):
    """Base class for other exceptions"""
    pass


class ValueTooSmallError(Error):
    """Raised when the input value is too small"""
    pass


class ValueTooLargeError(Error):
    """Raised when the input value is too large"""
    pass


# you need to guess this number
number = 10

# user guesses a number until he/she gets it right
while True:
    try:
        i_num = int(input("Enter a number: "))
        if i_num < number:
            raise ValueTooSmallError
        elif i_num > number:
            raise ValueTooLargeError
        break
    except ValueTooSmallError:
        print("This value is too small, try again!")
        print()
    except ValueTooLargeError:
        print("This value is too large, try again!")
        print()

print("Congratulations! You guessed it correctly.")