Just like different built-in exceptions, Python provides us the utility to create our own user defined exceptions or custom exceptions.

In all those scenarios where you need a specific exception to suite your purpose, we can use Custom Exception in Python.

Below are the steps to create a user-defined exception or custom exception in python,

1. Create a class derived from Exception class. This class will be your solution to create a custom exception.

Suggested Tutorial: Exception Handling in Python

class GiveException(Exception):
  def __init__(self, message):
    self.message = message

2. This Custom Exception is to be used when a developer feels there is need to do so. This Exception can be raised using raise keyword in Python.

raise GiveException('Hello')

3. This Exception can also handled using try-except block technique for Exception Handling.

try:
  // statements
except GiveException as msg:
  print(msg)

Let’s consider one example where we create our own exception for a banking system,

Problem: You are the admin of a MNC. Your task is to create a custom exception that should be raised when the paid leaves of an employee is less than 2.

Plan:

Create two .py files:
exception.py -> To define our exception
employee.py -> To implement our requirement

exception.py: This file defines our custom exception class. Currently, it just get’s initialized with a message.

class LeaveException(Exception):
    def __init__(self, message):
        print('XXXXXXXXXXXX LEAVE EXCEPTION RAISED XXXXXXXXXXXX')
        self.message = message
        # other statements

employee.py: This file contains the business logic.

# let's import our exception
from exception import LeaveException

class EmployeeLeaveData:
    def __init__(self, id, email, name, leaves_left):
        self.id = id
        self.email = email
        self.name = name
        self.leaves_left = leaves_left

    def validate_data(self):
        # raise the exception if leaves left are less than 2
        if self.leaves_left < 2:
            raise LeaveException(
                'Leave Balance for Employee {}-{} is less than minimum threshold.'.format(self.id, self.name))
        else:
            print('Leaves for Employee {}-{} are more than minimum threshold.'.format(self.id, self.name))


try:
    emp1 = EmployeeLeaveData(1231, '[email protected]', 'John Deol', 5)
    emp1.validate_data()
    emp2 = EmployeeLeaveData(1232, '[email protected]', 'Sunny Deol', 1)
    emp2.validate_data()
except LeaveException as l:
    print(l)

O/P

Leaves for Employee 1231-John Deol are more than minimum threshold.
XXXXXXXXXXXX LEAVE EXCEPTION RAISED XXXXXXXXXXXX
Leave Balance for Employee 1232-Sunny Deol is less than minimum threshold.

So, now we have created our own custom exception for our own purpose. Hope it was helpful tutorial for you.

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