Module 3 Lesson 8: Creating Custom Modules

In the previous lesson, we learned how to import modules like math and random. But you don't have to rely on Python's built-in tools. You can turn any Python file you write into a module that can be used by other parts of your program! This is the secret to building massive, professional applications.

Lesson Overview

In this lesson, we will cover:

• The Folder Structure: How to organize your files.
• Creating a Module: Saving functions in a separate file.
• Importing Your File: The path to modular code.
• The if __name__ == "__main__": block: Protecting your module from accidental execution.

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1. Creating Your First Module

Let's say you have a bunch of math helper functions. Instead of copying them into every new project, you can save them in a file named my_helpers.py.

File: my_helpers.py

def add(a, b):
    return a + b

def multiply(a, b):
    return a * b

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2. Using Your Module

Now, create a second file in the same folder called main.py. You can import your helpers just like you did with the math module!

File: main.py

import my_helpers

result = my_helpers.add(10, 5)
print(f"The total is {result}")

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3. The __name__ Protection

Sometimes you want a file to be both a standalone script AND a module. But if you have print statements in your module, they will run as soon as you import it. To prevent this, we use this special Python idiom:

def useful_func():
    print("Doing work...")

if __name__ == "__main__":
    # This block ONLY runs if you run this file directly.
    # It will NOT run if you import this file as a module.
    useful_func()

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4. Why Use Custom Modules?

1. Organization: Keep your "logic" separate from your "user interface."
2. Reusability: Use the same database_tools.py in 5 different projects.
3. Readability: No one wants to read a file with 10,000 lines of code. Split it into 10 files of 1,000 lines instead.

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Practice Exercise: The Geometry Library

1. Create a file named geometry.py.
2. Inside, define functions for area_of_circle(radius) and area_of_square(side).
3. Create a second file named app.py in the same folder.
4. In app.py, import your geometry module and ask the user for inputs to calculate areas.
5. Print the results.

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Quick Knowledge Check

1. How do you import a file named utils.py located in the same folder?
2. If you have a function test() inside your module, how do you call it from another file if you used import my_module?
3. What happens to code placed inside the if __name__ == "__main__": block when the file is imported?
4. Where should you store your custom modules so they are easy to find?

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Key Takeaways

• Any .py file can be a module.
• Modules allow you to share code between different files.
• Use the if __name__ == "__main__": block to keep your modules clean.
• Modular code is easier to maintain, test, and read.

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What’s Next?

We know how to make our own modules, but Python already comes with a treasure chest of them. In Lesson 9, we’ll deep-dive into the Python Standard Library to see what "batteries are included"!