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Building a Simple API

You can build APIs with plain Flask routes.

Example: in-memory todo API (learning)

from flask import Flask, request
 
app = Flask(__name__)
 
todos = [
    {"id": 1, "title": "Learn Flask", "done": False},
]
 
 
@app.get("/api/todos")
def list_todos():
    return {"items": todos}
 
 
@app.post("/api/todos")
def create_todo():
    payload = request.get_json(silent=True) or {}
    title = (payload.get("title") or "").strip()
 
    if not title:
        return {"error": "validation_error", "message": "title is required"}, 400
 
    new_id = max([t["id"] for t in todos], default=0) + 1
    todo = {"id": new_id, "title": title, "done": False}
    todos.append(todo)
 
    return todo, 201

from flask import Flask, request
 
app = Flask(__name__)
 
todos = [
    {"id": 1, "title": "Learn Flask", "done": False},
]
 
 
@app.get("/api/todos")
def list_todos():
    return {"items": todos}
 
 
@app.post("/api/todos")
def create_todo():
    payload = request.get_json(silent=True) or {}
    title = (payload.get("title") or "").strip()
 
    if not title:
        return {"error": "validation_error", "message": "title is required"}, 400
 
    new_id = max([t["id"] for t in todos], default=0) + 1
    todo = {"id": new_id, "title": title, "done": False}
    todos.append(todo)
 
    return todo, 201

Notes

.get_json(silent=True).get_json(silent=True) avoids raising exceptions on bad JSON
Always validate input
Use status codes consistently

Moving from in-memory to database

In real apps:

replace the list with SQLAlchemy models
implement to_dict()to_dict() methods
add pagination and authentication

🧪 Try It Yourself

Exercise 1 – Create a Flask App

Exercise 2 – Dynamic Route

Exercise 3 – Return JSON

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