14.5 Raising exceptions

Exception handling in Python is crucial for managing errors and unexpected situations. It allows programmers to gracefully handle issues, validate input, and create custom error types. This skill is essential for writing robust, user-friendly code.

Advanced exception handling concepts like exception hierarchies and context managers further enhance code reliability. By mastering these techniques, developers can create more resilient and maintainable Python programs, improving overall software quality and user experience.

Exception Handling

Raise statement for input validation

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• Manually raises exceptions in Python code to enforce specific conditions or constraints on user input

  • [raise](https://www.fiveableKeyTerm:raise) ExceptionType("Error message")

    is the syntax

  • raise [ValueError](https://www.fiveableKeyTerm:ValueError)("Invalid input. Please enter a positive number.")

    raises a ValueError if input is invalid
• Useful for indicating errors when user provides invalid input and raising appropriate exceptions
• Common exception types for invalid input:
  • ValueError raised when input is correct type but inappropriate value
  • TypeError raised when input is wrong data type
  • IndexError raised when index is out of range
• After raising an exception, program execution immediately transfers to nearest exception handler
  • If no suitable handler found, program terminates and displays error message (traceback)

Exception impact on execution

• Normal flow of program execution disrupted when exception is raised
• Program stops executing current code block and starts looking for exception handler
• Python searches for exception handler in this order:
  1. In same code block where exception occurred
  2. In enclosing code blocks moving upward in call stack
  3. If no suitable handler found, program terminates and displays error message
• If exception handler found, program execution transfers to that handler
  • Handler can be

    [try-except](https://www.fiveableKeyTerm:try-except)

    block or

    [try-except-finally](https://www.fiveableKeyTerm:try-except-finally)

    block
• After exception is handled, program continues executing from point immediately after exception handler
• If no exception raised, program execution continues normally without interruption

Custom exceptions in Python

• Define your own exception types specific to program's needs by creating custom exceptions
• Create custom exception by defining new class that inherits from built-in

  Exception

  class or one of its subclasses

  • class [CustomError](https://www.fiveableKeyTerm:CustomError)(Exception): pass

    is an example of defining a custom exception
• Custom exceptions can include additional attributes or methods to provide more information about the error

  • class [InvalidAgeError](https://www.fiveableKeyTerm:InvalidAgeError)(Exception): def __init__(self, age): self.age = age

    is an example with additional age attribute
• Raise custom exceptions using

  raise

  statement followed by instance of custom exception class

  • raise InvalidAgeError(age)

    raises the custom InvalidAgeError exception
• Handle custom exceptions by catching them using

  try-except

  block

  • try: ... except InvalidAgeError as e: print(f"Invalid age: {e.age}")

    catches and handles the custom exception
• Custom exceptions make code more readable and maintainable by providing specific error types for different scenarios
• Allow handling different types of errors differently and provide more informative error messages to user

Advanced Exception Handling Concepts

• Exception hierarchy: Built-in exceptions in Python form a hierarchy, with more specific exceptions inheriting from more general ones
• Exception chaining: Allows you to associate one exception with another, preserving the context of the original error
• Context manager: A protocol for resource management that ensures proper setup and cleanup, often used with the

  with

  statement
• Error handling best practices:
  • Only catch specific exceptions you can handle
  • Use finally blocks for cleanup operations
  • Avoid catching and re-raising exceptions without adding information