How To Use User Defined Exception In Python

Python provides a lot of built-in exception classes. All the built-in classes are sub-class of builtins.BaseException class. Besides the built-in exception class, you can also create customize exception classes. But all custom exception classes must extend builtins.Exception class. The builtins.Exception class is also a sub-class of builtins.BaseException class. This example will tell you how to define a custom python exception class and how to use it.

1. Create Custom Python Exception Class Example.

1. Create a PyDev project and a python package and a module com.dev2qa.example.exception.CustomException.py.(  You can refer article How To Run Python In Eclipse With PyDev )
2. Now edit the CustomException.py module file, and copy the below source code into it.

   '''
   Created on Jan 3, 2020
   
   @author: songzhao
   '''
   from builtins import Exception
   
   # The custom exception need to extend 
   class CustomException(Exception):
       
       def __init__(self, *params):
           
           if len(params) > 0:
               
               self.exception_message = params[0]
           
       
       def __str__(self):
           return 'This is a customized exception class, the error message is :' + self.exception_message
       
       
   # This function will raise exception by the input parameter value.
   # The input parameter is a boolean value. 
   def test_exception(ThrowCustomException):
       
       try:
           
           if(ThrowCustomException):
           
               # Raise an instance of CustomException class. 
               raise CustomException('This exception is created and threw in try block.', 'hello')
        
           else:
               # Below code will raise a ZeroDivisionError error.
               a = 1 / 0
               
       except CustomException as e:
               
               print('Exception class name : ', e.__class__.__name__)
               
               print('Exception data : ', e)       
       
       except ZeroDivisionError as e:
               
               print('Exception class name : ', e.__class__.__name__)
               
               print('Exception data : ', e)            
               
       finally:
               
               print('The example run complete.\n')            
                   
           
   if __name__ == '__main__':
       
       # Test system built-in exception.
       test_exception(False)
       
       # Test customize exception.
       test_exception(True)
   

3. Right-click the python source code, and click Run As —> Python Run menu item, then you can get the below output in the console. The first error is a system built-in error ZeroDivisionError. The second error is the CustomException error.

   Exception class name :  ZeroDivisionError
   Exception data :  division by zero
   The example run complete.
   
   Exception class name :  CustomException
   Exception data :  This is a customized exception class, the error message is :This exception is created and threw in try block.
   The example run complete.