Python User-defined Exceptions














































Python User-defined Exceptions




User-defined Exceptions



Python throws errors and exceptions, when there is a code gone wrong, which may cause program to stop abruptly. Python also provides exception handling method with the help of try-except. Some of the standard exceptions which are most frequent include IndexError, ImportError, IOError, ZeroDivisionError, TypeError and FileNotFoundError. A user can create his own error using exception class



Creating User-defined Exception:

Programmers may name their own exceptions by creating a new exception class. Exceptions need to be derived from the Exception class, either directly or indirectly. Although not mandatory, most of the exceptions are named as names that end in %u201CError%u201D similar to naming of the standard exceptions in python.


Deriving Error from Super Class Exception:

Super class Exceptions are created when a module needs to handle several distinct errors. One of the common way of doing this is to create a base class for exceptions defined by that module. Further, various subclasses are defined to create specific exception classes for different error conditions.


class Error(Exception):
 
    pass
  
class TransitionError(Error):
  
    # Raised when an operation attempts a state 
    # transition that's not allowed.
    def __init__(self, prev, nex, msg):
        self.prev = prev
        self.next = nex
  
      
        self.msg = msg
try:
    raise(TransitionError(2,3*2,"Not Allowed"))
  
except TransitionError as error:
    print('Exception occured: ',error.msg)



Output:

('Exception occured: ', 'Not Allowed')

How to use standard Exceptions as base class?


Runtime error is a class is a standard exception which is raised when a generated error does not fall into any category. This program illustrates how to use runtime error as base class and network error as derived class. In a similar way, any exception can be derived from the standard exceptions of Python.



class Networkerror(RuntimeError):
    def __init__(self, arg):
        self.args = arg
  
try:
    raise Networkerror("Error")
  
except Networkerror as e:
    print (e.args)


Output:

('E', 'r', 'r', 'o', 'r')



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