FastAPI Lab 3 – Build a Simple To-Do List API

Introduction

Have you ever wondered how apps like Google Keep , Todoist , or even your phone’s reminder app manage and organize your tasks behind the scenes? In this lab, you’ll step into the world of web APIs and build your very own Simple To-Do List API using FastAPI and Pydantic .

With this API, you’ll be able to add tasks , view all tasks , and understand how data flows between clients and servers — the same concepts used in real-world apps.

Don’t worry if you’ve never worked with APIs before — this guide is beginner-friendly , walks you through every step, and requires no prior experience. By the end, you’ll have a working backend that could be the foundation for your own productivity app!

Understanding the Requirements

You will build an API that:

Stores tasks in memory (no database required)
Allows adding tasks via POST requests
Allows viewing all tasks via GET requests
Uses Pydantic to validate incoming task data

Setting Up

Install FastAPI and Uvicorn:

pip install fastapi uvicorn

GET Requests

Purpose: Retrieve or fetch data from a server.
Effect on Server: Does not change server data (read-only).
Data Transmission: Data is sent via URL query parameters (visible in the URL).
Idempotence: Yes — making the same GET request multiple times produces the same result.
Caching: GET requests can be cached by browsers and proxies.
Use Case Examples:
Viewing all tasks in a to-do list API
Retrieving a user profile
Loading a web page

      
       @app.get("/tasks")
def get_tasks():
    return {"tasks": tasks}

Visiting /tasks in your browser or via curl retrieves all tasks.

POST Requests

Purpose: Send data to the server to create or update resources.
Effect on Server: Changes server state (e.g., adds a new record).
Data Transmission: Data is sent in the request body (not visible in the URL).
Idempotence: Usually no — repeating a POST request can create multiple resources.
Caching: POST requests are usually not cached.
Use Case Examples:
Adding a new task to a to-do list
Creating a new user account
Submitting a form

      
       @app.post("/add")
def add_task(item: Task):
    tasks.append(item.task)
    return {"message": "Task added!", "task": item.task}

Key Differences Between GET and POST

Feature	GET	POST
Purpose	Retrieve data	Send or create data
Alters Server Data	No	Yes
Data Location	URL query parameters	Request body
Visibility	URL visible	Body hidden
Idempotent	Yes	Usually No
Caching	Can be cached	Usually not cached

Summary:

Use GET to read data without changing anything.
Use POST to send data or make changes to the server.

Writing the To-Do API

Create a file named main.py and add:

      
       from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

# In-memory "database"
tasks = []

# Data model for creating tasks
class Task(BaseModel):
    task: str

@app.post("/add")
def add_task(item: Task):
    tasks.append(item.task)
    return {"message": "Task added!", "task": item.task}

@app.get("/tasks")
def get_tasks():
    return {"tasks": tasks}

Code Explanation

1. Importing Libraries

FastAPI → to create API endpoints
BaseModel → to define and validate request data

2. Creating the App

      
       app = FastAPI()

3. Creating an In-Memory Database

      
       tasks = []

tasks is a simple Python list that stores all task strings

4. Defining the Task Model

      
       class Task(BaseModel):
    task: str

Ensures that the incoming request contains a string field named task

5. Adding Tasks

      
       @app.post("/add")
def add_task(item: Task):
    tasks.append(item.task)
    return {"message": "Task added!", "task": item.task}

Accepts POST requests to /add
Validates the request body with Pydantic Task model
Adds the task to the in-memory list
Returns a JSON response confirming the addition

6. Retrieving All Tasks

      
       @app.get("/tasks")
def get_tasks():
    return {"tasks": tasks}

Accepts GET requests to /tasks
Returns all tasks stored in memory

Running the Server

python -m uvicorn main:app --reload

--reload automatically restarts the server when you edit the code

Testing Your API

Add a Task

      
       curl -X POST "http://127.0.0.1:8000/add" -H "Content-Type: application/json" -d '{"task":"Buy groceries"}'

Response:

      
       {
  "message": "Task added!",
  "task": "Buy groceries"
}

View All Tasks

http://127.0.0.1:8000/tasks