Python Tips - Create A User-defined Exception

  

Scenario

Python provides a bunch of built-in exceptions such as FileNotFoundError, IOError, ConnectionError, ZeroDivisionError, IndexError and etc. However, they belong to system or application exceptions. In business operation, some patterns are not allowed to happen, for example, an account balance can’t be minus. When that pattern occurs, we can raise a particular business logic exception so that the program can catch it and process it and continue with grace. Hence, we can define a dedicated exception class in response to this case.

Example

Here we assume the exception class holds basic information, an error code and an error message. Of course, you can customize it according to your requirements. Both code and message are not modifiable, but the class must provide a means that an external object or method can use to get their values. 

The codes are shown below.

# Define BizException:

class BizException(Exception):

    __code = 'E0001'

    __message = 'A business logic exception'

    @property

    def code(self):

        return self.__code

    @property

    def message(self):

        return self.__message

    def getcode(self):

        return self.__code

    def getmessage(self):

        return self.__message

    def __str__(self):

        return self.__code + ': ' + self.__message

# Test BizException

if __name__ == '__main__':

    try:

        ex = BizException()

        raise ex

    except BizException as ex:

        print(type(ex))

        print(ex.code, ex.message)

        print(str(ex))

# Output:

<class 'bizexception.BizException'>

E0001 A business logic exception

E0001: A business logic exception