Building a REST API with Python and Flask

Building a REST API with Python is one of the most useful skills a backend developer can have. APIs power everything from mobile apps to third-party integrations. Flask makes it surprisingly straightforward to build one. Let’s go from zero to a working API.

What Makes an API “RESTful”?

REST (Representational State Transfer) is a set of conventions for how web services communicate. A RESTful API uses HTTP methods to perform operations on resources:

• GET — read data
• POST — create data
• PUT/PATCH — update data
• DELETE — remove data

Each resource has its own URL (called an endpoint), like /users or /products/42.

Setting Up Your Flask API

Install Flask and create app.py:

from flask import Flask, jsonify, request

app = Flask(__name__)

# In-memory "database" for this example
books = [
    {"id": 1, "title": "Python Crash Course", "author": "Eric Matthes"},
    {"id": 2, "title": "Fluent Python", "author": "Luciano Ramalho"},
]

@app.route("/books", methods=["GET"])
def get_books():
    return jsonify(books)

@app.route("/books/<int:book_id>", methods=["GET"])
def get_book(book_id):
    book = next((b for b in books if b["id"] == book_id), None)
    if book is None:
        return jsonify({"error": "Book not found"}), 404
    return jsonify(book)

@app.route("/books", methods=["POST"])
def create_book():
    data = request.get_json()
    new_book = {"id": len(books) + 1, "title": data["title"], "author": data["author"]}
    books.append(new_book)
    return jsonify(new_book), 201

if __name__ == "__main__":
    app.run(debug=True)

jsonify() converts Python dictionaries to JSON responses. The 201 status code means “created” — returning the right status codes is part of good REST API design.

Testing Your REST API

You don’t need a frontend to test your API. Use curl in the terminal:

# Get all books
curl http://localhost:5000/books

# Create a new book
curl -X POST http://localhost:5000/books \
  -H "Content-Type: application/json" \
  -d '{"title": "Clean Code", "author": "Robert Martin"}'

Or install Postman for a graphical interface — it’s free and much easier for exploring APIs visually.

Conclusion

You now have a working REST API with Python and Flask — GET and POST endpoints, JSON responses, and proper status codes. From here, connect it to a real database like PostgreSQL, add authentication, and deploy it. Every web and mobile app you use relies on an API like this one.

Read next: Connecting Python to PostgreSQL with psycopg2

External resource: Flask — Quickstart