Chapter 7: Modules and Packages

In Python, modules and packages help organize code, promote reusability, and manage complexity in large projects. A module is a file containing Python code, and a package is a collection of modules organized in directories.

Modules

A module is simply a Python file with a .py extension. Modules can contain functions, classes, and variables that can be reused in other scripts.

Creating and Importing a Module

1. Create a Module: Save the following as mymodule.py:

   def greet(name):
       return f"Hello, {name}!"
   
   pi = 3.14159

2. Import the Module: Use the import keyword to use the module in another script.

   import mymodule
   
   print(mymodule.greet("Alice"))  # Output: Hello, Alice!
   print(mymodule.pi)  # Output: 3.14159

3. Import Specific Items: Use the from keyword to import specific elements.

   from mymodule import greet
   
   print(greet("Bob"))  # Output: Hello, Bob!

4. Alias Modules: Use the as keyword to give a module an alias.

   import mymodule as mm
   
   print(mm.pi)  # Output: 3.14159

Built-in Modules

Python comes with a rich standard library of built-in modules. Some commonly used modules include:

Module	Description
math	Mathematical functions and constants.
os	Interacting with the operating system.
sys	Accessing system-specific parameters.
random	Generating random numbers.
datetime	Working with dates and times.

Example:

import math

print(math.sqrt(16))  # Output: 4.0
print(math.pi)  # Output: 3.141592653589793

Packages

A package is a collection of related modules grouped into a directory. It contains an __init__.py file, which can be empty or include package initialization code.

Creating a Package

1. Create a directory named mypackage.

2. Add an empty __init__.py file to the directory.

3. Add modules to the package:

   • module1.py:

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

   • module2.py:

     def subtract(a, b):
         return a - b

4. Import and use the package:

   from mypackage import module1, module2
   
   print(module1.add(5, 3))  # Output: 8
   print(module2.subtract(5, 3))  # Output: 2

Nested Packages

Packages can contain sub-packages for better organization.

mypackage/
    __init__.py
    subpackage/
        __init__.py
        module.py

The __name__ Variable

Every Python module has a special variable __name__. When a module is run directly, __name__ is set to "__main__". This is useful for writing code that behaves differently when a module is imported versus executed directly.

Example:

# mymodule.py
if __name__ == "__main__":
    print("This script is being run directly.")
else:
    print("This script is being imported.")

Exercises

Exercise 1:

Create a module calculator.py with functions add, subtract, multiply, and divide. Use it in another script.

Solution:

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

def subtract(a, b):
    return a - b

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

def divide(a, b):
    if b != 0:
        return a / b
    return "Cannot divide by zero"

# main.py
import calculator

print(calculator.add(10, 5))  # Output: 15
print(calculator.divide(10, 0))  # Output: Cannot divide by zero

Exercise 2:

Create a package shapes with modules circle and rectangle. Implement functions to calculate the area of each shape.

Solution:

# shapes/circle.py
def area(radius):
    return 3.14 * radius ** 2

# shapes/rectangle.py
def area(length, width):
    return length * width

# main.py
from shapes import circle, rectangle

print(circle.area(5))  # Output: 78.5
print(rectangle.area(4, 6))  # Output: 24

Best Practices

1. Use meaningful names for modules and packages.

2. Avoid circular imports (modules importing each other).

3. Keep modules small and focused on a specific task.

4. Use packages to organize related functionality.

In the next chapter, we will explore file handling, including reading, writing, and managing files in Python.