14.4 Handling exceptions

Exception handling in Python lets programs respond to errors without stopping abruptly. With try, except, else, and finally, you can handle missing files, invalid indexes, and other problems in a controlled way.

This section covers common file-related exceptions, try/except blocks for built-in exceptions, and advanced techniques for writing more reliable Python programs.

Exception Handling in Python

Common file reading exceptions

• FileNotFoundError raised when attempting to open a non-existent or inaccessible file (incorrect file path or deleted file)
• IndexError raised when trying to access an out-of-range index for a sequence (list, tuple, or string) can occur when iterating over lines in a file and attempting to access an index beyond the end of the file

Try/except for built-in exceptions

• Try/except statements handle exceptions and prevent program termination
  • Code that may raise an exception placed inside the try block
  • Exception handling code written in the corresponding except block
• Syntax:

  </>Python
  try:
      # Code that may raise an exception
  except ExceptionType:
      # Exception handling code

• Multiple except blocks can handle different types of exceptions each except block specifies the exception type it handles
• Optional else block can be added after the except block(s) code in the else block executed if no exceptions raised in the try block
• Optional finally block can be added after the except and else blocks code in the finally block always executed, regardless of whether an exception was raised or not
• The raise keyword can be used to manually trigger an exception

Error handling for file operations

• Use try/except statements to handle file-related exceptions
  • Wrap file operations (opening, reading, writing) inside a try block
  • Handle specific exceptions (FileNotFoundError, IndexError) in the corresponding except blocks
• Provide informative error messages to the user
  • In the except block, print a user-friendly error message indicating the nature of the exception
  • Use the str() function to convert the exception object to a string for display
• Close the file in the finally block
  • Ensure that the file is properly closed, regardless of whether an exception was raised or not
  • Use the close() method to close the file
• Example:

  </>Python
  try:
      file = open("data.txt", "r")
      # File operations (reading, processing)
  except FileNotFoundError:
      print("File not found. Please check the file path.")
  except IndexError:
      print("Invalid index accessed while reading the file.")
  finally:
      file.close()

Advanced Exception Handling

• Exception hierarchy: Python's exceptions are organized in a hierarchical structure, with more specific exceptions inheriting from more general ones
• Custom exceptions: Developers can create their own exception classes by inheriting from built-in exception classes
• Exception chaining: Allows one exception to be raised in response to another, preserving the original exception's traceback
• Traceback: Provides a detailed report of the call stack at the point where an exception occurred, useful for debugging