Exception Handling and Custom Exceptions

35 min

Python uses try-except blocks to handle exceptions gracefully, allowing your program to recover from errors instead of crashing. Exception handling is crucial for robust applications that need to deal with unexpected situations like file not found errors, network failures, or invalid user input. Proper exception handling improves user experience and makes debugging easier.

The try-except-else-finally structure provides comprehensive error handling. The `try` block contains code that might raise an exception, `except` catches specific exceptions, `else` runs when no exception occurs, and `finally` always executes for cleanup operations. This structure ensures your code handles all scenarios properly.

You can create custom exceptions by inheriting from the `Exception` class or its subclasses. Custom exceptions make error handling more specific and meaningful, allowing you to catch and handle different error types appropriately. They also improve code readability and debugging by providing domain-specific error types.

Exception chaining allows you to preserve the original exception context when raising new exceptions. Using `raise ... from ...` creates a chain that shows both the original and new exceptions, making debugging much easier when exceptions occur deep in call stacks. This is especially useful when transforming exceptions.

The `finally` block is always executed, whether an exception occurs or not. This makes it perfect for cleanup operations like closing files, releasing locks, or cleaning up resources. It ensures your code always performs necessary cleanup, even when errors occur, preventing resource leaks.

Best practices for exception handling include being specific about which exceptions you catch, not catching all exceptions with bare `except:`, and providing meaningful error messages. Logging exceptions is also important for debugging production issues. Understanding the exception hierarchy helps you catch exceptions at the right level.

Key Concepts

try-except blocks handle exceptions gracefully.
Custom exceptions inherit from Exception class for specific error types.
finally block always executes for cleanup operations.
Exception chaining preserves original exception context.
Be specific about which exceptions to catch, avoid bare except clauses.

Learning Objectives

Master

Using try-except-else-finally for comprehensive error handling
Creating custom exception classes for specific error types
Understanding exception hierarchy and inheritance
Implementing proper exception chaining and error messages

Develop

Defensive programming and error handling thinking
Understanding when and how to raise exceptions
Designing robust error handling strategies

Tips

Be specific about which exceptions you catch—avoid bare except clauses.
Create custom exceptions for domain-specific errors to improve code clarity.
Use finally blocks for cleanup operations that must always execute.
Log exceptions with meaningful context for debugging production issues.

Common Pitfalls

Catching all exceptions with bare except:, hiding bugs and making debugging impossible.
Swallowing exceptions silently without logging or handling them properly.
Creating overly broad exception handlers that catch too many exception types.
Not using finally blocks for cleanup, causing resource leaks.

Summary

Exception handling allows programs to recover from errors gracefully.
Custom exceptions make error handling more specific and meaningful.
finally blocks ensure cleanup operations always execute.
Proper exception handling is essential for robust, production code.

Exercise

Create a custom exception and use it in a function.

class ValidationError(Exception):\n  pass\n\ndef validate_age(age):\n  if age < 0:\n    raise ValidationError("Age cannot be negative")\n  if age > 150:\n    raise ValidationError("Age seems unrealistic")\n  return True\n\ntry:\n  validate_age(-5)\nexcept ValidationError as e:\n  print(f"Validation error: {e}")

Exercise Tips

Add more specific exception classes (NegativeAgeError, UnrealisticAgeError).
Use exception chaining with 'raise ... from ...' to preserve context.
Add a finally block to demonstrate cleanup operations.
Implement __str__ method in custom exceptions for better error messages.

Exception Handling and Custom Exceptions