Lecture 02/27: CSC1302

Exception Handling in Python

• Overview of Exception Handling

  • Focus on handling exceptions across multiple methods (method chaining).

  • Identify responsible method for exception handling during errors in a chain of function calls.

• Example Method Calls

  • Function f() calls g(), which calls h().

  • An error occurring in h() needs to be handled.

• Responsibility for Handling Exceptions

  • The responsibility can be assigned to any function in the call stack:

    • Handle it in h().

    • Delegate to g().

    • Delegate to f().

  • If no handling code exists, Python searches upward in the call stack.

• Error Demonstration

  • Example: f(a, b) where a=1 and b=0 results in a zero division error.

• Three Options for Handling Exceptions

1. Inside the function where the error occurs (h()).

2. In the calling method (g()).

3. In the outer caller (f()).

• If not handled, it raises an error.

Exception Block Structure

• Try, Except, Finally Blocks

  • try block: Executes code that may raise an exception.

  • except block: Executes code to handle exceptions.

  • finally block: Always executes, used for cleanup (e.g., closing files).

• Use Case for finally

  • Important in file I/O operations to ensure files are always closed properly.

Custom Exceptions

• Creating a Custom Exception

  • Define a new class inheriting from Exception.

  • Example: A custom exception for negative integers: LessThanZeroError.

  • Raise this exception when a negative input is encountered.

• Code Structure for Custom Exception

  • Ask user for input.

  • Raise LessThanZeroError if input is negative.

Modules in Python

• Purpose of a Module

  • Avoid writing repetitive code by encapsulating commonly used functions in a single file.

  • Modules can be imported into scripts or other modules.

• Importing Modules

  • To import a module named my_module.py, use: import my_module.

  • Python has a built-in variable sys.modules to show loaded modules.

• Using Functions from Modules

  • Use module_name.function_name() to access specific functions.

  • Option to import specific functions: from my_module import function_name.

• Dependencies

  • A module required by another program is referred to as a dependency.

  • Python checks built-in modules first, then searches paths defined in sys.path.

• Reloading Modules

  • If changes to a module are made, use reload(module_name) to update without restarting the program.

Packages vs. Modules

• Concept of a Package

  • A package is a collection of related modules organized in directories.

  • Example structure:

    • game/

      • sound/

      • graphics/

  • Importing modules:

    • Import specific modules using the dot notation: import game.sound.music.

Common Python Modules

• Examples of Useful Modules

  • datetime, random, math, sys.

  • Reference additional examples in Ziboch, Chapter 11.

File Input/Output (I/O) Operations

• File System Structure

  • Types of file paths:

    • Absolute Path: Full path from root.

      • Example: /users/username/documents/file.txt.

    • Relative Path: Path from the current directory.

• Reading/Writing Files

  • General steps for file handling:

    1. Open the file using open().

    2. Perform read/write operations.

    3. Close the file.

• Opening Files

  • open(filename, mode):

    • Modes:

      • r (read), w (write), a (append), etc.

• Cursor Position

  • r, w, and r+ modes start at the file beginning.

  • a mode starts at the end of the file.

File Reading Techniques

• Reading Methods

  • read(size): Reads a specified number of characters.

  • readlines(): Reads all lines into a list.

• String Handling

  • Use split() to convert a string to a list of words.

Writing to Files

• Write Operations

  • write(content): Writes content—overwrites existing content in w mode; adds in a mode.

• New Lines

  • Use \n for new lines in text files.

Using with Statement

• Benefits of with

  • Automatically closes files once the block ends, preventing errors of forgetting to close files.

Upcoming Topics

• Discuss command line arguments and their usage in the next session.