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Modules and Packages

As your codebase grows, you should organize your code into multiple files (Modules) and directories (Packages) to keep it clean and maintainable.

1. What is a Module?

A Module is simply a Python file (ending in .py) that contains executable statements, function definitions, or class definitions.

Example: Creating and Importing a Module

Suppose you create a file named mymath.py with the following content:

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

def square(x):
    return x ** 2

You can now use this module in another Python file (main.py) in the same directory:

# main.py

# Method 1: Import the whole module
import mymath
print(mymath.add(5, 10))  # Output: 15

# Method 2: Import specific functions
from mymath import square
print(square(4))  # Output: 16

# Method 3: Import and rename (alias)
import mymath as mm
print(mm.add(1, 2))  # Output: 3

Note: Avoid using from module import * as it imports all names into the current namespace, which can lead to name clashes and makes code hard to read.

2. The __name__ == "__main__" Guard

When a Python file is run directly, its special global variable __name__ is set to "__main__". If the file is imported by another script, __name__ is set to the file's name.

Using this guard allows you to write testing code that only runs when you execute the file directly, and does not run when someone imports your module.

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

# This code block will NOT run when main.py imports mymath
if __name__ == "__main__":
    print("Testing mymath module directly:")
    print(add(10, 20))

3. What is a Package?

A Package is a folder containing multiple Python modules. Packages allow you to group related modules together in a directory structure.

Package Directory Structure

To make Python treat a directory as a package, it must contain a file named __init__.py (this file can be completely empty, but it is required).

myproject/
│
├── main.py
└── mypackage/
    ├── __init__.py
    ├── geometry.py
    └── algebra.py

Importing from a Package

You can import modules from packages using dot notation:

# In main.py
from mypackage import geometry
from mypackage.algebra import add

4. Absolute vs. Relative Imports

  • Absolute Imports: Specify the full path to the module from the project's root folder. Highly readable and preferred. 
    from mypackage.algebra import add
  • Relative Imports: Use dot . notation to import relative to the current module's location.
    • . means the current directory.
    • .. means the parent directory.
    from .algebra import add  # Import from algebra module in same directory

Further Reading