Creating Custom Modules

A Module in Python is simply a file ending in .py. While every script you write is technically a module, professional development involves splitting your logic into many modules to keep code clean, testable, and reusable.

When you import a file, you aren’t just linking to it—you are executing it to create a new Namespace.

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1. Module Basics

Let’s create a utility module for a hypothetical mathematical library.

geometry.py

import math

# A constant
PI = math.pi

# Functions
def area_of_circle(radius):
    return PI * (radius ** 2)

def circumference(radius):
    return 2 * PI * radius

You can now use this in another file:

app.py

import geometry

print(geometry.area_of_circle(5))

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2. The Entry Point: if __name__ == "__main__"

When you import geometry, Python executes every line of geometry.py. If you had a print("Starting...") at the top level of geometry.py, it would run as soon as you imported it.

To prevent code (like tests or scripts) from running during an import, we use the __name__ check.

How it Works:

• If you run python geometry.py, Python sets __name__ to "__main__".
• If you import geometry, Python sets __name__ to "geometry".

geometry.py (Updated)

def area_of_circle(radius):
    return 3.14 * (radius ** 2)

if __name__ == "__main__":
    # This block ONLY runs if you execute this file directly.
    # Excellent for unit tests or manual verification.
    print("Testing geometry module...")
    assert area_of_circle(1) == 3.14
    print("Test passed!")

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3. Under the Hood: The Module Namespace

When you import a module, Python creates a Module Object. This object has its own __dict__ attribute, which stores all the functions, classes, and variables defined in the file.

Name Conflicts

Because each module has its own namespace, you can have a function named calculate() in module_a.py and another named calculate() in module_b.py without them ever clashing. You access them as module_a.calculate() and module_b.calculate().

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4. Searching for Modules: sys.path

When you say import my_lib, Python looks in a specific list of directories. If it can’t find it, it raises ModuleNotFoundError.

The Order of Search

1. The Input Script Directory: The folder where the script you are running is located.
2. Standard Library: Built-in modules like os and sys.
3. Site-Packages: Third-party libraries installed via pip.

check_path.py

import sys
# sys.path is just a list of strings.
# You can actually append to it at runtime to import from
# unusual locations, though this is rarely recommended.
print(sys.path)

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5. Summary Table: Module Best Practices

Rule	Reason
Use Snake Case	my_module.py is standard; MyModule.py is not.
Avoid Library Names	Naming your file math.py or random.py will break imports.
Use Docstrings	Add a triple-quoted string at the very top of the module.
Keep Logic in Functions	Avoid “Global” code that runs immediately upon import.