Creating Modules

You've used modules made by others. Now it's time to create your own! Creating modules helps you organize code, avoid repetition, and share functionality across projects.

The great news: creating a module is incredibly simple – just create a Python file!

Creating Your First Module

Any .py file is a module! Create greetings.py:

# greetings.py
"""Module for greeting functions."""

def say_hello(name):
    """Return a hello greeting."""
    return f"Hello, {name}!"

def say_goodbye(name):
    """Return a goodbye message."""
    return f"Goodbye, {name}!"

# Constants
DEFAULT_NAME = "World"

Now use it in another file:

# main.py
import greetings

print(greetings.say_hello("Alice"))
print(greetings.DEFAULT_NAME)

The name == 'main' Pattern

This pattern lets a file work both as a module AND as a standalone script:

# calculator.py
def add(a, b):
    return a + b

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

# This only runs when executed directly
if __name__ == "__main__":
    # Test code
    print(f"2 + 3 = {add(2, 3)}")
    print(f"2 × 3 = {multiply(2, 3)}")

                    How __name__ Works                            

                                                                  
   When you RUN the file directly:                               
   $ python calculator.py                                        
   __name__ = "__main__"  → Test code RUNS                       
                                                                  
   When you IMPORT the file:                                     
   import calculator                                             
   __name__ = "calculator"  → Test code SKIPPED                  
                                                                  
   This lets you include tests/demos that don't run on import!   
                                                                  

Module Best Practices

# utils.py
"""
Utility functions for data processing.

This module provides helper functions for common
data manipulation tasks.
"""

__all__ = ['process_data', 'clean_text']  # Public API

# Public functions
def process_data(data):
    """Process and return cleaned data."""
    return _validate(data)

def clean_text(text):
    """Remove extra whitespace from text."""
    return ' '.join(text.split())

# Private function (starts with _)
def _validate(data):
    """Internal validation - not exported."""
    return data if data else []

# Module-level code
_cache = {}  # Private module variable

Key Takeaways

                   Remember These Points                          

                                                                  
   Any .py file is automatically a module                      
                                                                  
   if __name__ == "__main__":                                  
     Code here only runs when file executed directly              
                                                                  
   Add docstrings for documentation                            
                                                                  
   _underscore names are "private" (convention)                
                                                                  
   __all__ controls what 'import *' exports