4 - Modules, File I/O, and Error Handling — Topics & Learning Outcomes

Module Topics

Introduction to Python Modules

• What Is a Module? — A module is a single Python file containing definitions, functions, classes, and variables that can be reused in other scripts.
• Importing Modules with import — The import statement loads a module into the current namespace, making its contents accessible via dot notation.
• Selective Imports with from…import — The from…import syntax lets you import specific names from a module directly into the current namespace.
• Module Aliases — The 'as' keyword creates a shorter or more descriptive alias for an imported module or name.
• The Module Search Path — When Python encounters an import statement it searches a sequence of directories defined in sys.path to locate the module file.

Writing and Using Custom Modules

• Creating a Custom Module File — A custom module is created by writing functions, classes, or constants in a .py file and saving it in a location accessible to your scripts.
• The __name__ == '__main__' Guard — This guard allows a module file to contain both reusable code and a standalone test or demo block that only executes when the file is run directly.
• Organizing Functions and Constants — Group related functions and constants logically within a module to make the interface clear to other developers and scripts.
• Importing Your Custom Module — Once saved in the correct directory, a custom module is imported using the same syntax as any standard library module.
• Documenting a Custom Module — Good documentation within a module enables other engineers to understand and correctly use its functions without reading the implementation.

Using Standard Library Modules

• The math Module — The math module provides mathematical functions and constants commonly needed in engineering calculations.
• The os and os.path Modules — The os module provides operating-system-independent functions for interacting with the file system and environment.
• The sys Module — The sys module exposes interpreter-level variables and functions, including command-line arguments and the module search path.
• The datetime Module — The datetime module supplies classes for manipulating dates and times, which is useful for timestamping engineering data logs.
• The random Module — The random module generates pseudo-random numbers and provides tools for random sampling, useful in simulations and testing.

File I/O with the with Statement

• Opening Files and File Modes — Python's built-in open() function returns a file object that provides methods for reading and writing; the mode argument determines how the file is accessed.
• The with Statement Context Manager — The with statement ensures a file is automatically closed when the indented block exits, even if an exception occurs.
• Reading Text Files — File objects provide multiple methods for reading text content depending on whether you need the entire file, one line, or all lines as a list.
• Writing Text Files — Opening a file in write or append mode allows you to output strings to the file using the write or writelines methods.
• Working with File Paths Using pathlib — The pathlib module provides an object-oriented interface for constructing and manipulating file paths that works consistently across operating systems.

Reading and Writing CSV Files

• What Is a CSV File? — A CSV (Comma-Separated Values) file stores tabular data as plain text, with each row on a new line and columns separated by a delimiter, typically a comma.
• Reading CSV Files with csv.reader — csv.reader wraps a file object and returns an iterator that yields each row as a list of strings.
• Reading CSV Files with csv.DictReader — csv.DictReader reads each row as an ordered dictionary, using the header row as keys, which makes column access by name more readable.
• Writing CSV Files with csv.writer — csv.writer provides methods to write rows to a CSV file, handling quoting and delimiter escaping automatically.
• Writing CSV Files with csv.DictWriter — csv.DictWriter writes dictionaries as CSV rows, with field names specified at construction time.

Working with JSON

• What Is JSON? — JSON (JavaScript Object Notation) is a lightweight, text-based format for representing structured data using key-value pairs, arrays, and nested objects.
• Parsing JSON with json.loads and json.load — Python's json module converts a JSON string or file into native Python data structures using the loads and load functions.
• Serializing Data with json.dumps and json.dump — The json module converts Python objects into JSON-formatted strings or writes them to a file using dumps and dump.
• JSON for Configuration Files — Storing simulation parameters or tool settings in a JSON file allows engineers to modify inputs without editing source code.
• Limitations of Python's json Module — Not all Python objects can be directly serialized to JSON; certain types require custom handling or conversion before they can be encoded.

Error Handling with try-except

• What Is an Exception? — An exception is a runtime error that interrupts normal program flow; if not handled, it terminates the script and displays a traceback.
• Basic try-except Structure — A try block contains code that might raise an exception; the except block defines how to respond if a specific exception type is raised.
• The else and finally Clauses — The optional else clause runs only if no exception was raised; the finally clause runs unconditionally and is used for cleanup actions.
• Accessing Exception Information — Using 'as' in an except clause binds the exception object to a variable, providing access to its message and attributes for logging or debugging.
• Raising Exceptions — The raise statement lets you deliberately trigger an exception when your code detects an invalid condition, signaling errors clearly to calling code.
• Practical Error Handling in Engineering Scripts — Combining try-except with file I/O, user input, and numeric operations produces robust scripts that handle real-world data quality issues gracefully.

Student Learning Outcomes

By the end of this module, students will be able to:

Course Outcomes (reference)