SRJahirTech

Python Full Tutorial -- Part 10: Modules & Packages

By Suraj Ahir 2026-01-06 11 min read

Python's module and package system is one of its greatest strengths. The standard library covers an enormous range of tasks with no extra installation. The PyPI ecosystem adds hundreds of thousands more packages. Understanding how to organise your own code into modules and how to use pip effectively is essential professional Python knowledge.

Creating and Importing Modules

# utils.py
def calculate_tax(amount, rate=0.18):
    return amount * rate

SUPPORTED_CURRENCIES = ["INR", "USD", "EUR"]

# main.py
import utils
tax = utils.calculate_tax(1000)       # 180.0

from utils import calculate_tax
tax = calculate_tax(1000, rate=0.28)  # 280.0

import utils as u
print(u.SUPPORTED_CURRENCIES)

Package Structure

myapp/
    __init__.py          # Marks directory as package
    models/
        __init__.py
        user.py
        product.py
    services/
        __init__.py
        payment.py
    utils/
        validators.py
    main.py

from myapp.models.user import User
from myapp.services.payment import process_payment
from myapp.utils import validators

Standard Library Highlights

import os, sys, json, re, datetime
from pathlib import Path
from collections import Counter, defaultdict
import logging

# os
os.makedirs("logs", exist_ok=True)
files = os.listdir(".")
env_var = os.environ.get("HOME", "/tmp")

# datetime
now = datetime.datetime.now()
formatted = now.strftime("%Y-%m-%d %H:%M")
tomorrow = now + datetime.timedelta(days=1)

# re (regular expressions)
pattern = r"^[\w.-]+@[\w.-]+\.\w+$"
print(bool(re.match(pattern, "suraj@example.com")))  # True

Virtual Environments and pip

# Create virtual environment
python3 -m venv venv
source venv/bin/activate        # Linux/macOS
.\venv\Scripts\activate         # Windows

# Install packages in venv
pip install requests flask pandas pytest

# Save dependencies
pip freeze > requirements.txt

# Install from file
pip install -r requirements.txt

# Useful commands
pip list                   # List installed
pip show requests          # Info about package
pip install --upgrade pip  # Update pip itself

Frequently Asked Questions

In Part 11, we cover Object-Oriented Programming -- classes, inheritance, and the OOP patterns used in major Python frameworks.

Written by

Suraj Ahir

Cloud & DevOps engineer running four live production services on my own AWS infrastructure. I write everything on this site myself — no ghostwriters, no AI filler.

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Virtual Environments with pyenv

# Install pyenv (manages multiple Python versions)
curl https://pyenv.run | bash

# Install Python versions
pyenv install 3.11.8
pyenv install 3.12.2

# Set default globally
pyenv global 3.11.8

# Set per project
cd myproject
pyenv local 3.12.2   # Creates .python-version file

# Create virtual environment with specific version
pyenv local 3.12.2
python -m venv venv
source venv/bin/activate

Project Management with Poetry

pip install poetry

# Create new project
poetry new myproject
cd myproject

# Add dependencies
poetry add flask requests boto3
poetry add --group dev pytest black mypy

# pyproject.toml manages everything
[tool.poetry.dependencies]
python = "^3.11"
flask = "^3.0"
requests = "^2.31"

# Install all dependencies
poetry install

# Run commands in virtual env
poetry run python app.py
poetry run pytest

# Export to requirements.txt for compatibility
poetry export -f requirements.txt --output requirements.txt

Module Best Practices for Large Projects

myapp/
    __init__.py          # Keep minimal: just expose public API
    config.py            # Configuration constants
    models/
        __init__.py      # from .user import User (re-export)
        user.py
        order.py
    services/
        __init__.py
        email_service.py
        payment_service.py
    utils/
        __init__.py
        validators.py
        formatters.py
    tests/
        conftest.py      # Shared pytest fixtures
        test_models.py
        test_services.py

# models/__init__.py
from .user import User
from .order import Order, OrderStatus
from .product import Product

__all__ = ["User", "Order", "OrderStatus", "Product"]

# Now users can do:
from myapp.models import User, Order   # Clean import
# Instead of:
from myapp.models.user import User      # Exposes internal structure

Import System Deep Dive

import sys

# See where Python looks for modules
print(sys.path)
# ['', '/usr/lib/python3.11', '/usr/lib/python3.11/lib-dynload',
#  '/home/suraj/.local/lib/python3.11/site-packages', ...]

# Add a directory to search path
sys.path.insert(0, '/home/suraj/mylibs')

# See all cached (already imported) modules
print(list(sys.modules.keys())[:10])

# Reload a module after changing it (useful in REPL)
import importlib
importlib.reload(mymodule)

# Conditional imports
try:
    import ujson as json  # Faster JSON if available
except ImportError:
    import json           # Fall back to stdlib

# Lazy imports for startup performance
def get_pandas():
    import pandas as pd   # Only imported when first called
    return pd

Creating Executable Python Scripts

#!/usr/bin/env python3
"""
disk-check: Check disk usage and alert if above threshold.
Usage: disk-check --threshold 85 --path /
"""
import argparse
import sys
import shutil

def check_disk(path: str, threshold: int) -> int:
    total, used, free = shutil.disk_usage(path)
    pct = (used / total) * 100
    
    print(f"Path: {path}")
    print(f"Total: {total / 1e9:.1f} GB")
    print(f"Used:  {used  / 1e9:.1f} GB ({pct:.1f}%)")
    print(f"Free:  {free  / 1e9:.1f} GB")
    
    if pct >= threshold:
        print(f"ALERT: Disk usage {pct:.1f}% exceeds threshold {threshold}%")
        return 1
    return 0

def main():
    parser = argparse.ArgumentParser(description="Check disk usage")
    parser.add_argument("--path",      default="/",  help="Path to check")
    parser.add_argument("--threshold", type=int, default=85, help="Alert %")
    args = parser.parse_args()
    sys.exit(check_disk(args.path, args.threshold))

if __name__ == "__main__":
    main()