Python Checkpoint: Turn a Script into a Small Package

This guide explains checkpoint: turn a script into a small package through a focused Python example. You will learn the behavior to expect, why it matters in a real program, and how to recognize the common mistakes before applying it in your own project.

What You Will Learn

This checkpoint turns a single-file calculator into a small maintainable project: one module handles calculations, another is the entry point and a fresh environment stores its dependencies.

When and Why to Use It

The following example focuses on a practical use of Checkpoint: Turn a Script into a Small Package.

Step-by-Step Example

def calculate_subtotal(price, quantity):
    if quantity < 0:
        raise ValueError("Quantity cannot be negative")
    return price * quantity

def run():
    print(f"Subtotal: {calculate_subtotal(5, 4):.2f}")

if __name__ == "__main__":
    run()

Understanding the Example

The output is Subtotal: 20.00. Moving calculate_subtotal to a separate module would preserve its API while keeping run in the entry file.

Expected Output

Subtotal: 20.00

Important Behavior and Edge Cases

A package is not complete just because it has multiple folders. Verify imports, tests and environment instructions from a clean working directory.

Try It Yourself

Create a two-module project with a README, a simple test and a .gitignore.

Run environment-management commands in your terminal, not in a Python source file. Use the executable associated with the selected project interpreter and verify the environment before installing dependencies.

Common Beginner Mistakes

• A package is not complete just because it has multiple folders. Verify imports, tests and environment instructions from a clean working directory.
• Running installation commands against a different Python interpreter than the one used for the project.
• Skipping the practice step and assuming the example covers every input. Test at least one normal case and one edge case relevant to this topic.

Key Takeaways

• This checkpoint turns a single-file calculator into a small maintainable project: one module handles calculations, another is the entry point and a fresh environment stores its dependencies.
• The output is Subtotal: 20.00.
• Apply the technique deliberately, checking its documented input requirements and failure behavior before integrating it into a larger project.