How would i make a custom error message in python

Question

In a python program I am making, I want it to only take integers, and if it gets a string say "There has been an error in the system." instead of murmering sensless information the user will not understand

Answer 1

Use a try-except block to capture the error and use the raise statement to say the error message of your choice:

try:
    a = int(input())
except:
    raise Exception('There has been an error in the system')

Answer 2

Using try, except and raise

Since ValueError inherits from the Exception class, the first parameter when creating a ValueError object is the message it prints:

try:
    int("string") #the code that raises the error
except ValueError:
    raise ValueError("Your custom message here.")

This prints:

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ValueError: invalid literal for int() with base 10: 'string'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
ValueError: Your custom message here.

If you don't want the previous error chain to print, put from None in the raise statement:

try:
    int("string") #the code that raises the error
except ValueError:
    raise ValueError("Your custom message here.") from None

This prints:

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
ValueError: Your custom message here.

I suggest you leave the chain because it gives more information, like what was inputted that raised the error. If you want to include the information from the original error in the custom message, use the error's attributes:

from traceback import format_tb

try:
    int("string") #the code that raises the error
except ValueError as err:
    raise ValueError("Custom message with traceback and original message\n" + format_tb(err.__traceback__)[0] + err.args[0] + "\nEnd of error message.") from None

This prints

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
ValueError: Custom message with traceback and message
  File "<stdin>", line 2, in <module>
invalid literal for int() with base 10: 'string'
End of error message.

Though this allows for customization of the error's print, the code is a little unpythonic.

Using assert

Because in the question you said you wanted to block all strings, you can use assert and isinstance():

obj = "a string" #the input you want to raise the error on
assert not isinstance(obj, str), "Your custom message here."

This prints:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError: Your custom message here.

Though using assert looks clean, the error won't carry as much information because it would be a generic AssertionError. Raising a ValueError tells more information about what caused the error at a glance.

Answer 3

You need to use a try except block to catch the error - see the documentation. Then you could just print a message, and, if necessary, exit the program:

try:
    value = int(input("Enter an integer: "))
except ValueError:
    print("There has been an error in the system.")
    input()    # To let the user see the error message
    # if you want to then exit the program
    import sys
    sys.exit(1)

Answer 4

If you want to make an error, you use raise. Here is an example:

raise SyntaxError('MUHAHA THIS IS A ERROR')

Answer 5

If you do not want to add another indentation level by using a try-except block, you can change the handling of all errors by adding the following to the beginning of your code:

import sys
def my_except_hook(exctype, value, traceback):
        print('There has been an error in the system')
sys.excepthook = my_except_hook

In case of an error, only your specified error message is printed out. In addition, this prevents the stack trace from being displayed.

Answer 6

Use raise Exception('There has been an error in the system') may be useful for most cases but you may also need create meaningful errors for specific systems.

Eg.

class ExpiredTaxIdException(Exception):
    def __init__(self):
        Exception.__init__(self, 'Tax ID expired')

Then you can call it on your code, sample:

from your_execeptions_file import ExpiredTaxIdException

class ClientsController:
    def is_profile_valid(client):
        if client.tax_id.is_valid == False:
            raise ExpiredTaxIdException()
        return True

# eg. you call it on your API on another place of your code that can't accept invalid tax_id

try:
    ClientsController().verify_client(client)
except ExpiredTaxIdException as e:
    return {'error': str(e)}

>>> { "error": "Tax ID expired" }

Answer 7

You can try this..

import ctypes
ctypes.windll.user32.MessageBoxW(None, u"CUSTOM MESSAGE", u"TITLE BAR", 0)