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Modules & Packages

Real-world Python projects aren’t single files. They are split across multiple files (modules) and directories (packages). Understanding how to organize code and manage dependencies is crucial.

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1. Modules

A Module is simply a Python file (.py). Any Python file can be imported by another. 
# math_utils.py
def add(a, b):
    return a + b

PI = 3.14159

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Importing

You can import the whole module or specific parts. 
# main.py

# Option 1: Import entire module
import math_utils
print(math_utils.add(1, 2))

# Option 2: Import specific items (Cleaner)
from math_utils import add, PI
print(add(1, 2))

# Option 3: Alias (Common for libraries like pandas as pd)
import math_utils as mu
print(mu.PI)

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__name__ == "__main__"

This is a common idiom. It checks if the file is being run directly (like python main.py) or imported. 
if __name__ == "__main__":
    print("This code runs ONLY if you execute this file directly.")
    print("It does NOT run if you import this file.")

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2. Packages

A Package is a directory containing Python modules. Historically, it required a __init__.py file (though Python 3.3+ makes this optional, it’s still good practice). 
my_package/
    __init__.py  # Marks this folder as a package
    module1.py
    module2.py

from my_package import module1
from my_package.module2 import some_function

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3. The Standard Library

Python is famous for being “Batteries Included”. It has a massive standard library built-in.

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pathlib (Modern File Paths)

Stop using os.path.join. Use pathlib. It works on Windows, Mac, and Linux seamlessly. 
from pathlib import Path

# Create path
p = Path("data") / "file.txt"

# Read text
if p.exists():
    content = p.read_text()

# Find files
for file in Path(".").glob("*.py"):
    print(file)

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json (Data Serialization)

JSON is the language of the web. Python handles it natively. 
import json

data = {"name": "Alice", "age": 30}

# Serialize to String
json_str = json.dumps(data)

# Save to File
with open("data.json", "w") as f:
    json.dump(data, f)

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datetime (Dates & Times)

from datetime import datetime, timedelta

now = datetime.now()
tomorrow = now + timedelta(days=1)

print(now.strftime("%Y-%m-%d"))

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4. Virtual Environments (venv)

The Golden Rule of Python: Never install packages globally. Always use a virtual environment. A virtual environment creates an isolated folder for your project’s dependencies. This prevents “Dependency Hell” where Project A needs Library v1.0 and Project B needs Library v2.0.

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Setup

# 1. Create the environment (run once)
# This creates a folder named 'venv'
python -m venv venv

# 2. Activate it
# Windows:
venv\Scripts\activate
# Mac/Linux:
source venv/bin/activate
Once activated, your terminal prompt will change (e.g., (venv) C:\Project>). Now, when you run pip install, packages go into this folder, not your system.

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5. Package Management (pip)

pip is the package installer for Python. It fetches packages from PyPI (Python Package Index). 
# Install a package
pip install requests

# List installed packages
pip list

# Save your dependencies to a file
pip freeze > requirements.txt

# Install dependencies from a file (Crucial for collaboration)
pip install -r requirements.txt

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Example: Using requests

requests is the most popular Python library. It makes HTTP requests simple. 
import requests

response = requests.get("https://api.github.com")
print(response.status_code)
print(response.json())

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Summary

  • Modules: .py files.
  • Packages: Folders with __init__.py.
  • Standard Library: Learn pathlib, json, datetime.
  • venv: Always isolate your projects.
  • pip: The tool to install external libraries.
Next, we’ll tackle Advanced Python concepts like decorators and async programming.