Modules & Packages - Backend Development with FastAPI

1. What is a Module?

A module is simply a Python file (with a .py extension) containing variables, functions, or classes that you want to reuse in other parts of your program.

Creating and Importing a Module

Suppose we 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

Now, we can use these functions in another file (e.g., main.py) in the same folder by importing it:

# main.py
import calculator

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

2. What is a Package?

A package is a directory (folder) that contains multiple modules. It allows you to organize related modules hierarchically.

The Role of `init.py`

To turn a standard folder into a Python package, it must contain a file named __init__.py. This file does several things:

Marks Directories: It tells Python that this directory is an importable package.
Initialization Code: Python executes __init__.py first when the package or any sub-module is imported.
Creates Public APIs: You can import sub-modules inside __init__.py to make them accessible directly from the package root:
# mypackage/__init__.py from .utils import format_text
Now consumers can write:
from mypackage import format_text
Controls Wildcard Imports: Define a list called __all__ to restrict what gets imported when someone uses from package import *:
__all__ = ['utils']

3. Python’s Standard Library Essentials

Python follows a “batteries included” philosophy, providing powerful built-in modules that are ready to use immediately.

A. The `pathlib` Module

Modern Python uses pathlib for object-oriented filesystem paths:

from pathlib import Path

# Get home directory or current directory
current_dir = Path.cwd()
file_path = current_dir / "data" / "users.json"

# Check if path exists
print(file_path.exists())

# Read and write text easily
if file_path.exists():
    content = file_path.read_text()

B. The `os` Module

Used for interacting with the operating system:

import os

# Get current working directory
print(os.getcwd())

# Get environment variables
database_url = os.environ.get("DATABASE_URL", "sqlite:///local.db")

C. The `datetime` Module

Provides classes for manipulating dates and times:

import datetime

# Current date and time
now = datetime.datetime.now()
print(now)

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

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

D. The `uuid` Module

Generates RFC 4122 compliant universally unique identifiers (UUIDs):

import uuid

# Generate a random UUID (UUID4)
unique_id = uuid.uuid4()
print(unique_id)  # e.g., "d3b07384-d113-4956-a5cc-484d2d476100"

E. The `json` Module

JSON is the standard data format for web APIs. Python’s json module parses and serializes JSON:

import json

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

# Serialization (Dict -> JSON String)
json_str = json.dumps(data)

# Deserialization (JSON String -> Dict)
parsed_data = json.loads(json_str)

4. Third-Party Packages (e.g., `requests`)

While the Standard Library provides urllib for handling HTTP requests, the community standard is the third-party requests library due to its clean, human-readable API. To use it, you must first install it within your virtual environment:

pip install requests

Making HTTP Requests

Here is how you can perform typical GET and POST operations using requests:

import requests

# 1. 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"])

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

​1. What is a Module?

​Creating and Importing a Module

​2. What is a Package?

​The Role of __init__.py

​3. Python’s Standard Library Essentials

​A. The pathlib Module

​B. The os Module

​C. The datetime Module

​D. The uuid Module

​E. The json Module

​4. Third-Party Packages (e.g., requests)

​Making HTTP Requests