14.4 Handling exceptions

Exception handling in Python is crucial for managing errors gracefully. It allows programmers to anticipate and handle potential issues, preventing abrupt program termination and improving user experience.

This section covers common file-related exceptions, try/except blocks for built-in exceptions, and advanced techniques. By mastering these concepts, you'll write more robust and user-friendly Python programs that can handle unexpected situations effectively.

Exception Handling in Python

Common file reading exceptions

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• 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:

  try:
      # Code that may raise an exception
  except [ExceptionType](https://www.fiveableKeyTerm:ExceptionType):
      # Exception handling code
  

• Multiple

  except

  blocks can handle different types of exceptions each

  except

  block specifies the exception type it handles
• Optional

  [else](https://www.fiveableKeyTerm: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](https://www.fiveableKeyTerm: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:

  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