Pythonic Way to create a Error Class from Exception

Question

I am working on a Project where I want to raise a error and I have been creating class each time I need a new Exception. I am staying away from generic / builtin errors as they are less descriptive for the purpose I need them for. I came up with a solution but I am not sure if it is a pythonic way to create an instance.

def CustomError(name: str, message: str):
    def constructor(self, msg=message):
        self.message = msg
        return

    def repr_method(self):
        return self.message

    error = type(name,
                 (Exception, object),
                 {'__init__': constructor,
                  '__str__': repr_method})
    return error(msg=message)

This does the trick as I can raise this error with a custom name and message instead of creating a new class every time. Please can someone let me know if this actually is a pythonic way if not then what is the recommended way?

I am expecting someone to confirm that this is actually a valid way of using higher order functions for creating a class instance or point me to a recommended way of doing it.

You are raising errors that can't be caught. Exceptions tend to be more generic. Or if fine grained, inherit from some a more generic type. if you have more than a dozen exception types in a package, doesn't it start being a burden to the user? And a dozen are easy to enter manually. — tdelaney
– tdelaney, Commented Nov 29, 2022 at 15:21

Gameplay · Accepted Answer · 2022-11-29 15:15:38Z

3

I wouldn't say it's a pythonic way to be honest. Python has initializers, not "constructors", the constructor would be the magic method __new__ if you really needed that (which is rare). For customizable initializers the decorator @classmethod (multiple factory methods) is the way to go. Also __str__ and __repr__ are not the same (neither is their purpose).

Also most often you actually should define a new class for each type of custom exception (and inherit from the Exception class).

class MyCustomException(Exception):
    def __init__(self, name:str, message:str)->None:
        super().__init__(message) 
        self.name = name

answered Nov 29, 2022 at 15:15

Gameplay

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