User-defined Exceptions in Python with Examples

Last Updated : 04 Jan, 2025

In Python, exceptions are used to handle errors that occur during the execution of a program. While Python provides many built-in exceptions, sometimes we may need to create our own exceptions to handle specific situations that are unique to application. These are called user-defined exceptions.

To manage errors and exceptions, Python offers an exception handling mechanism using try and except blocks. Some common exceptions include IndexError, ImportError, IOError, ZeroDivisionError, TypeError and FileNotFoundError.

Creating User-Defined Exceptions

User-defined exceptions are created by defining a new class that inherits from Python’s built-in Exception class or one of its subclasses. By doing this, we can create custom error messages and handle specific errors in a way that makes sense for our application.

Steps to Create and Use User-Defined Exceptions

Define a New Exception Class: Create a new class that inherits from Exception or any of its subclasses.
Raise the Exception: Use the raise statement to raise the user-defined exception when a specific condition occurs.
Handle the Exception: Use try-except blocks to handle the user-defined exception.

Example of a User-Defined Exception:

Python

# Step 1: Define a custom exception class class InvalidAgeError(Exception):     def __init__(self, age, msg="Age must be between 0 and 120"):         self.age = age         self.msg = msg         super().__init__(self.msg)      def __str__(self):         return f'{self.age} -> {self.msg}'  # Step 2: Use the custom exception in your code def set_age(age):     if age < 0 or age > 120:         raise InvalidAgeError(age)     else:         print(f"Age set to: {age}")  # Step 3: Handling the custom exception try:     set_age(150)  # This will raise the custom exception except InvalidAgeError as e:     print(e)

Output

150 -> Age must be between 0 and 120

Explanation:

InvalidAgeError class inherits from Exception. It has an __init__ method that takes an age and an optional message. __str__ method returns a string that will be shown when the exception is printed.
In the set_age function, we check if the age is outside the valid range (0–120). If it is, we raise the InvalidAgeError.
We use a try-except block to catch and handle the InvalidAgeError. When this error is raised, the error message is printed.

Customizing Exception Classes

When we create a custom exception, we subclass Python’s built-in Exception class (or a subclass like ValueError, TypeError, etc.). We can then add our own attributes, methods or custom logic to make our exception more informative.

Python

# Step 1: Subclass the Exception class  class InvalidAgeError(Exception):     def __init__(self, age, msg="Age must be between 0 and 120", error_code=1001):         # Custom attributes         self.age = age         self.msg = msg         self.error_code = error_code         super().__init__(self.msg)  # Call the base class constructor      # Step 2: Customize the string representation of the exception          def __str__(self):         return f"[Error Code {self.error_code}] {self.age} -> {self.msg}"  # Step 3: Raising the custom exception  def set_age(age):     if age < 0 or age > 120:         raise InvalidAgeError(age)     else:         print(f"Age set to: {age}")  # Step 4: Handling the custom exception with additional information  try:     set_age(150)  # This will raise the custom exception except InvalidAgeError as e:     print(e)

Output

[Error Code 1001] 150 -> Age must be between 0 and 120

Explanation:

__str__ method is overridden to provide a custom error message when the exception is printed. Message includes the error code and age, making it more informative.
In the set_age function, if the age is invalid, the custom exception InvalidAgeError is raised. Exception is then caught in the try-except block, and the customized error message is printed.

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.

Python

# 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)