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Apart from using the built-in exception classes, we could even create our own exception classes and once we have created our own exception class, we will be able to raise and catch our own exception just as we raise and catch the built-in exceptions. Let's see how to get it done.

How to create our own exception class

In order to create our own exception classes, we simply just have to create a class which inherits any inbuilt exception class. For example -

• A simple way to create an exception class is by creating a class which inherits the general superclass of all the exception classes(except system exiting exception classes) i.e. Exception class, or


• We could even create an exception class based on a specific exception class i.e. by creating a class which inherits any specific subclass of Exception class, such as NameError, BufferError, TypeError, OverflowError, ZeroDivisionError, etc.

• Creating our own exception class by inheriting the Exception class.



• In the upcoming code, we are creating our own exception class by inheriting the general superclass i.e. Exception class, which is the mother of all the exception classes(except system exiting exception classes).
• Next, we will throw our exception out of a try block and catch it in the except block.



import sys


#Defining our exception class named MyException, inheriting from Exception class
class  MyException(Exception):
    pass

try :
    raise MyException('User-Defined Exception')
except MyException as e :
    print("Exception caught : ", e)
	
    #Calling exc_info() function gives the name of caught exception
    print("More information about exception : ", sys.exc_info()) 





Output




Exception caught :  User-Defined Exception
More information about exception :  (<class '__main__.MyException'>, MyException('User-Defined Exception'), <traceback object at 0x0000000002EF8308>)








• User Defined Exception by extending a specific NameError class



• In the upcoming code, we are creating our own exception class by inheriting a specific exception class - NameError.
• Next, we will throw our exception out of a try block and catch it in the except block.



import sys


#Defining our exception class named MyException2, inheriting from NameError class
class  MyException2(NameError):
    pass

try :
    raise MyException2('Another User-Defined Exception')
except MyException2 as e :
    print("Exception caught : ", e)
	
    #Calling exc_info() function gives the name of caught exception
    print("More information about exception : ", sys.exc_info()) 





Output




Exception caught :  Another User-Defined Exception
More information about exception :  (<class '__main__.MyException2'>, MyException('User-Defined Exception'), <traceback object at 0x0000000002EF8308>)






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• Using the raise keyword to throw a user-defined exception.



• we are creating our own exception class by inheriting the general mother of all exception classes i.e. - Exception
• Next, we will use the raise keyword, to explicitly raise our exception out of the function, from where our defined exception was generated.



#Python - A User Defined Exception Example 


class myException2(Exception):
    acc_bal = 0 		#current account balance

    
    
    def withdraw(self, debitMoney):
        if(self.acc_bal>1000):    #if account balance is greater than 1000$ then you can dedit money
            acc_bal = acc_bal - debitMoney
        else:	# if account balance is less than 1000$ then debiting throws our user defined exception.
            raise myException2('Cannot debit. Low Account Balance')


ob= myException2()
try:
	ob.withdraw(100)
except myException2 as e :
	print('Exception handled - ', e)





Output




Exception handled -  Cannot debit. Low Account Balance



Here in the code above, we have made our own exception class, myException3 by extending Exception class. This class throws our defined exception, i.e. myException2, which was already caught in the except block.





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