Python Modules, Packages & the Standard Library Guide

Create modules and packages, use essential Standard Library tools like pathlib and json, and install third-party packages with pip.

As your Python programs grow, keeping everything in a single file becomes unmanageable. Modules let you split code across multiple files, and packages let you organise related modules into directories. Python’s “batteries included” philosophy means the Standard Library ships with powerful modules for file paths, dates, UUIDs, JSON, and more — all ready to use without installing anything extra. This page shows you how to organise your code effectively and leverage the tools Python provides out of the box.

What is a Module?

A module is simply a Python file (.py) containing variables, functions, or classes that you want to reuse across your project.

Create a file named calculator.py:

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

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

PI = 3.14159

Import and use it from another file in the same directory:

# main.py
import calculator

result = calculator.add(5, 3)
print(result)           # 8
print(calculator.PI)    # 3.14159

What is a Package?

A package is a directory containing multiple modules. To turn a folder into a Python package, add an __init__.py file inside it.

The Role of __init__.py

__init__.py serves four purposes:

1. Marks the directory as an importable package.

2. Runs initialisation code when the package is first imported.

3. Creates a public API by re-exporting sub-modules:

   # mypackage/__init__.py
   from .utils import format_text
   

   Consumers can now write from mypackage import format_text instead of the full path.

4. Controls wildcard imports using __all__:

   __all__ = ["utils"]
   

Python’s Standard Library Essentials

pathlib — Modern File Paths

Use pathlib for object-oriented, cross-platform path handling:

from pathlib import Path

current_dir = Path.cwd()
file_path = current_dir / "data" / "users.json"

print(file_path.exists())   # True or False

if file_path.exists():
    content = file_path.read_text()

os — Operating System Interface

import os

print(os.getcwd())   # Current working directory

# Safely read an environment variable with a fallback
database_url = os.environ.get("DATABASE_URL", "sqlite:///local.db")

datetime — Dates and Times

import datetime

now = datetime.datetime.now()
print(now)

# Format to string
formatted = now.strftime("%Y-%m-%d %H:%M:%S")

# Parse from string
parsed = datetime.datetime.strptime("2026-07-20", "%Y-%m-%d")

uuid — Unique Identifiers

import uuid

unique_id = uuid.uuid4()
print(unique_id)   # e.g. d3b07384-d113-4956-a5cc-484d2d476100

json — JSON Serialisation

import json

data = {"name": "Alice", "role": "admin"}

# Dict → JSON string
json_str = json.dumps(data)
print(json_str)   # {"name": "Alice", "role": "admin"}

# JSON string → Dict
parsed_data = json.loads(json_str)
print(parsed_data["name"])   # Alice

string — String Constants

The string module provides useful predefined character sets:

Constant	Contents
string.ascii_lowercase	a-z
string.ascii_uppercase	A-Z
string.ascii_letters	a-zA-Z
string.digits	0-9
string.punctuation	!@#$…

import string

print(string.ascii_letters)   # abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ
print(string.digits)          # 0123456789
print(string.punctuation)     # !"#$%&'()*+,-./:;<=>?@[\]^_`{|}~

Generate a random password:

import random
import string

characters = string.ascii_letters + string.digits + string.punctuation
password = "".join(random.choice(characters) for _ in range(12))
print(password)

Third-Party Packages — requests

While Python’s urllib handles HTTP, the community standard is the requests library for its clean, human-friendly API.

Install it inside your virtual environment first:

pip install requests

Making HTTP Requests

import requests

# GET request
response = requests.get("https://jsonplaceholder.typicode.com/posts/1")
if response.status_code == 200:
    post_data = response.json()
    print("Title:", post_data["title"])

# POST request
payload = {
    "title": "Hello FastAPI",
    "body": "Learning about modules and packages",
    "userId": 1
}
post_response = requests.post(
    "https://jsonplaceholder.typicode.com/posts",
    json=payload,
    headers={"Content-Type": "application/json"}
)
print("Status Code:", post_response.status_code)   # 201 Created
print("Response JSON:", post_response.json())

Always check response.status_code before accessing response.json(). A successful response returns 200 for GET and 201 for POST. Use response.raise_for_status() to automatically raise an exception for 4xx/5xx responses.