How To Use User Defined Exception In Python

Python provide a lot of built-in exception classes. All the built-in classes are sub class of builtins.BaseException class. Besides built-in exception class, you can also create customize exception class. But all custom exception class 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 )
    pydev project python exception example
  2. Now edit the CustomException.py module file, and copy 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 below output in console. The first error is a system built-in error ZeroDivisionError. The second error is 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.
    
    
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