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User-defined Exceptions in Python with Examples

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  • Difficulty Level : Medium
  • Last Updated : 30 Aug, 2022
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Prerequisite: This article is an extension to Exception Handling.

In this article, we will try to cover How to Define Custom Exceptions in Python with Examples. 

Example: 

class CustomError(Exception):
    pass

raise CustomError("Example of Custom Exceptions in Python")

Output: CustomError: Example of Custom Exceptions in Python

Python throws errors and exceptions when the code goes wrong, which may cause the program to stop abruptly. Python also provides an 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.

User-Defined Exception in Python

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 “Error” similar to the naming of the standard exceptions in python. For example,

Python3




# A python program to create user-defined exception
# class MyError is derived from super class Exception
class MyError(Exception):
 
    # Constructor or Initializer
    def __init__(self, value):
        self.value = value
 
    # __str__ is to print() the value
    def __str__(self):
        return(repr(self.value))
 
 
try:
    raise(MyError(3*2))
 
# Value of Exception is stored in error
except MyError as error:
    print('A New Exception occurred: ', error.value)


Output

('A New Exception occurred: ', 6)

Customizing Exception Classes

To know more about class Exception, run the code below 

Python3




help(Exception)


Output

Help on class Exception in module exceptions:

class Exception(BaseException)
 |  Common base class for all non-exit exceptions.
 |  
 |  Method resolution order:
 |      Exception
 |      BaseException
 |      __builtin__.object
 |  
 |  Methods defined here:
 |  
 |  __init__(...)
 |      x.__init__(...) initializes x; see help(type(x)) for signature
 |  
 |  ----------------------------------------------------------------------
 |  Data and other attributes defined here:
 |  
 |  __new__ = <built-in method __new__ of type object>
 |      T.__new__(S, ...) -> a new object with type S, a subtype of T
 |  
 |  ----------------------------------------------------------------------
 |  Methods inherited from BaseException:
 |  
 |  __delattr__(...)
 |      x.__delattr__('name') <==> del x.name
 |  
 |  __getattribute__(...)
 |      x.__getattribute__('name') <==> x.name
 |  
 |  __getitem__(...)
 |      x.__getitem__(y) <==> x[y]
 |  
 |  __getslice__(...)
 |      x.__getslice__(i, j) <==> x[i:j]
 |      
 |      Use of negative indices is not supported.
 |  
 |  __reduce__(...)
 |  
 |  __repr__(...)
 |      x.__repr__() <==> repr(x)
 |  
 |  __setattr__(...)
 |      x.__setattr__('name', value) <==> x.name = value
 |  
 |  __setstate__(...)
 |  
 |  __str__(...)
 |      x.__str__() <==> str(x)
 |  
 |  __unicode__(...)
 |  
 |  ----------------------------------------------------------------------
 |  Data descriptors inherited from BaseException:
 |  
 |  __dict__
 |  
 |  args
 |  
 |  message

Example 1: User-Defined class with Multiple Inheritance

In the below article, we have created a class named “Error” derived from the class Exception. This base class is inherited by various user-defined classes to handle different types of python raise an exception with message

Python3




# define Python user-defined exceptions
class Error(Exception):
    """Base class for other exceptions"""
    pass
 
class zerodivision(Error):
    """Raised when the input value is zero"""
    pass
 
try:
    i_num = int(input("Enter a number: "))
    if i_num == 0:
        raise zerodivision
except zerodivision:
    print("Input value is zero, try again!")
    print()


Output

Enter a number: Input value is zero, try again!
()
Enter a number: Input value is zero, try again!

Example 2: Deriving Error from Super Class Exception

Superclass Exceptions are created when a module needs to handle several distinct errors. One of the common ways 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.

Python3




# class Error is derived from super class Exception
class Error(Exception):
 
    # Error is derived class for Exception, but
    # Base class for exceptions in this module
    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
 
        # Error message thrown is saved in msg
        self.msg = msg
 
try:
    raise(TransitionError(2, 3*2, "Not Allowed"))
 
# Value of Exception is stored in error
except TransitionError as error:
    print('Exception occurred: ', error.msg)


Output

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

How to use standard Exceptions as a base class?

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

Python3




# NetworkError has base RuntimeError
# and not Exception
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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