Before we dive into building a small application, let's take a moment to consolidate the fundamental concepts you've learned throughout this course. Understanding how these pieces fit together is essential for writing effective Python programs. This review will reinforce the building blocks we'll use shortly in our practical project.

Core Programming Elements

At the heart of any Python program are the ways we store and manipulate information.

Variables: You learned to use variables as named containers for storing data values. Assigning a value is straightforward, like user_name = "Alice" or item_count = 5. Python figures out the data type automatically (dynamic typing).
Data Types: We covered the essential built-in data types:
- Numbers: Integers (int for whole numbers, e.g., 42) and Floating-Point Numbers (float for numbers with decimals, e.g., 3.14).
- Text: Strings (str for sequences of characters, enclosed in quotes, e.g., "Hello, Python!").
- Booleans: (bool representing logical states: True or False).
Operators: These are the symbols that perform operations on data:
- Arithmetic: For calculations (+, -, *, /, % for modulus, // for floor division, ** for exponentiation).
- Comparison: For comparing values (==, !=, <, >, <=, >=), resulting in a Boolean True or False.
- Logical: For combining Boolean expressions (and, or, not).
Input/Output: You learned how to interact with the user using input() to get data and print() to display information. Remember that input() always returns a string, so you often need type conversion (e.g., int(), float()) using functions like int() or float().

Controlling the Flow

Programs rarely execute instructions straight through. You learned how to control the order of execution:

Conditional Statements: Using if, elif (else if), and else, you can make decisions in your code, executing specific blocks only when certain conditions are met.
Loops: For repeating actions:
- while loops continue executing a block as long as a condition remains True.
- for loops iterate over the items in a sequence (like a list, tuple, or string) or other iterable objects.
Loop Control: The break statement allows you to exit a loop prematurely, while continue skips the rest of the current iteration and proceeds to the next.

Organizing Data with Collections

When dealing with multiple pieces of related data, Python's collection types are indispensable:

Lists (list): Ordered, mutable (changeable) sequences of items. Defined with square brackets []. Great for collections where items might be added, removed, or changed.
Tuples (tuple): Ordered, immutable (unchangeable) sequences of items. Defined with parentheses (). Useful for fixed collections of related data where order matters.
Dictionaries (dict): Collections of key-value pairs. Defined with curly braces {} (e.g., {'name': 'Bob', 'age': 30}). Provide fast lookups based on keys. In modern Python versions, dictionaries maintain insertion order.
Sets (set): Unordered collections of unique items. Also defined with curly braces {} (but without key-value pairs, e.g., {1, 2, 3}). Efficient for membership testing and removing duplicates.

Building Reusable Code with Functions

Functions allow you to package blocks of code for reuse, making programs modular and easier to manage:

Defining Functions: Using the def keyword, followed by the function name, parentheses () for parameters, and a colon :.
Parameters and Arguments: Parameters are variables listed inside the parentheses in the function definition. Arguments are the actual values passed to the function when it's called.
Return Values: The return statement sends a value back from the function to the caller. If omitted, the function returns None.
Scope: Variables defined inside a function are local by default, meaning they only exist within that function. Variables defined outside are global.
Docstrings: Triple-quoted strings ("""Docstring goes here""") immediately after the def line are used to document what a function does.
Lambda Functions: We briefly saw lambda functions for creating small, anonymous functions inline.

Interacting with Files

Programs often need to read data from or write data to files:

Opening Files: Using the open() function, specifying the file path and a mode (like 'r' for read, 'w' for write, 'a' for append).
Reading and Writing: Using methods like .read(), .readline(), .readlines() to get data from files, and .write() to put data into files.
Closing Files: It's important to close files using .close() or, preferably, use the with statement (with open(...) as f:) which handles closing automatically, even if errors occur.

Modularity: Modules and Packages

To organize larger projects and leverage existing code:

Modules: Python files (.py) containing definitions and statements.
Importing: Using import module_name or from module_name import specific_item to use code defined in other modules.
Standard Library: Python comes with a vast standard library full of useful modules (e.g., math, datetime, random, os).
External Packages: Using tools like pip to install and use third-party packages developed by the wider Python community (found on the Python Package Index - PyPI).

Introduction to Object-Oriented Programming (OOP)

We touched upon the basic ideas of OOP:

Classes: Blueprints or templates for creating objects. Defined using the class keyword.
Objects (Instances): Specific instances created from a class. They hold their own data (attributes) and can perform actions (methods).
Attributes: Variables associated with an object, storing its state.
Methods: Functions defined inside a class that operate on an object's data.
__init__ Method: A special method (constructor) called automatically when a new object is created, used to initialize its attributes.
self Parameter: The conventional name for the first parameter in instance methods, representing the object instance itself.

Handling Errors and Exceptions

Programs can encounter errors during runtime. Exception handling allows you to manage these situations gracefully:

Exceptions: Errors detected during execution (e.g., ValueError, TypeError, FileNotFoundError).
try...except Blocks: The try block contains code that might raise an exception. If an exception occurs, the corresponding except block is executed.
Specific Exceptions: You can catch specific types of exceptions for tailored error handling.
else and finally: The optional else block runs if no exceptions occur in the try block. The finally block always runs, regardless of exceptions, useful for cleanup actions (like closing files).
Raising Exceptions: You can use the raise statement to signal an error condition in your own code.

This whirlwind tour refreshes the concepts you've mastered. Each of these plays a role in constructing Python applications, from the smallest scripts to complex systems. With these fundamentals firmly in mind, you are well-prepared to apply them in building the command-line tool in the upcoming sections.