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User-defined Exceptions in Python with Examples

Last Updated : 22 Jul, 2025

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.

• 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:

# 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)

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.

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.

# 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)

[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.

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.

# 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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