A proposal about exception subclassing

exception_subclass.py

#---- current behaviour 

# class UsernameTooShortError(Exception):
#     MSG = 'Username too short'
#     pass

# def add_user(username=None):
#     if len(username) < 4:
#         raise UsernameTooShortError()

# try:
#     username = 'ted'
#     print ("Adding a user", username)
#     add_user(username=username)
# except UsernameTooShortError as exc:
#     print("Error", exc.MSG)

# recomended exception message using str()
# we loose the message
# except UsernameTooShortError as exc:
#     print("Error", str(exc))


# proposal

# class UsernameTooShortError(Exception):
#     MSG = 'Username too short'

#     def __str__(self):
#         return str(self.MSG)

# def add_user(username=None):
#     if len(username) < 4:
#         raise UsernameTooShortError()

# try:
#     username = 'ted'
#     print ("Adding a user", username)
#     add_user(username=username)
# except UsernameTooShortError as exc:
#     print("Error", str(exc))


# abstraction
class BaseError(Exception):
    def __init__(self, message=None, *args, **kwargs):
        self.message = message

    def __str__(self):
        default = self.__class__.__name__
        if hasattr(self, 'MSG'):
            default = getattr(self, 'MSG')
        message = self.message or default
        return str(message)

class UsernameTooShortError(BaseError):
    pass
    #MSG = 'Username too short'


def add_user(username=None):
    if len(username) < 4:
        #raise UsernameTooShortError(f"Username {username} it's too short")
        raise UsernameTooShortError()

try:
    username = 'ted'
    print ("Adding a user", username)
    add_user(username=username)
except UsernameTooShortError as exc:
    print("Error", str(exc))
    #print("Error", str(exc.MSG))