Introduction
In Python, exceptions are events that disrupt the normal flow of a program. When such an event occurs, the program crashes unless the exception is handled. Python has many built-in exceptions, such as ValueError
, TypeError
, and IndexError
. However, there are situations where these built-in exceptions may not adequately represent the specific error conditions of your program. In such cases, you can create your own user-defined exceptions.
Creating User-Defined Exceptions
To create a user-defined exception in Python, you need to define a new class that inherits from the built-in Exception
class or one of its subclasses. By inheriting from Exception
, your custom exception class will be compatible with Python’s exception handling framework.
Here is the basic syntax for creating a user-defined exception:
class MyCustomError(Exception):
"""Base class for other exceptions"""
pass
Example 1: Basic User-Defined Exception
Let’s create a simple custom exception to handle errors when a specific condition is not met.
class NegativeNumberError(Exception):
"""Exception raised for errors in the input if the number is negative."""
def __init__(self, number, message="Number cannot be negative"):
self.number = number
self.message = message
super().__init__(self.message)
# Example of usage
def check_positive(number):
if number < 0:
raise NegativeNumberError(number)
try:
check_positive(-5)
except NegativeNumberError as e:
print(f"Error: {e}")
Example 2: Hierarchy of User-Defined Exceptions
Sometimes, it is beneficial to create a hierarchy of exceptions, especially when you want to handle different types of errors in different ways.
class CustomError(Exception):
"""Base class for other exceptions"""
pass
class InputError(CustomError):
"""Exception raised for errors in the input."""
def __init__(self, expression, message="Invalid input"):
self.expression = expression
self.message = message
super().__init__(self.message)
class CalculationError(CustomError):
"""Exception raised for errors in the calculation."""
def __init__(self, calculation, message="Error in calculation"):
self.calculation = calculation
self.message = message
super().__init__(self.message)
# Example of usage
def process_input(input_value):
if not isinstance(input_value, int):
raise InputError(input_value, "Input must be an integer")
def calculate_division(a, b):
if b == 0:
raise CalculationError(f"{a} / {b}", "Division by zero")
return a / b
try:
process_input("ten")
except InputError as e:
print(f"Input Error: {e}")
try:
calculate_division(10, 0)
except CalculationError as e:
print(f"Calculation Error: {e}")
Handling User-Defined Exceptions
User-defined exceptions are handled in the same way as built-in exceptions. You can use a try-except
block to catch and handle these exceptions.
try:
# Code that might raise an exception
process_input("ten")
except InputError as e:
print(f"Caught an InputError: {e}")
except CalculationError as e:
print(f"Caught a CalculationError: {e}")
except CustomError as e:
print(f"Caught a CustomError: {e}")
Adding Custom Methods to Exceptions
You can add custom methods to your exception classes to provide additional functionality.
class InvalidAgeError(Exception):
def __init__(self, age, message="Age must be between 0 and 120"):
self.age = age
self.message = message
super().__init__(self.message)
def log_error(self):
print(f"Logging error: Age {self.age} is not valid.")
# Example of usage
def set_age(age):
if age < 0 or age > 120:
error = InvalidAgeError(age)
error.log_error()
raise error
try:
set_age(150)
except InvalidAgeError as e:
print(f"Error: {e}")
Conclusion
User-defined exceptions provide a way to create more specific and meaningful error messages in your programs. By defining your own exceptions, you can handle error conditions in a more precise and readable manner. Remember to always document your custom exceptions so that their purpose is clear to anyone reading your code.
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