A Beginner’s Guide to Building a REST API with Flask and SQLAlchemy

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Building a REST API is a common task in modern web development. REST APIs allow for communication between a client (such as a web or mobile app) and a server to perform operations like retrieving, creating, updating, or deleting data. In this guide, we’ll walk through how to create a simple REST API using Flask, a lightweight web framework in Python, and SQLAlchemy, a powerful Object Relational Mapper (ORM).

Prerequisites

Before we begin, ensure you have Python installed on your system. You can check this by running:

bash

python --version

Step 1: Setting Up the Environment

First, let's create a virtual environment for our project and install the necessary dependencies.

bash

# Create a project folder and navigate into it mkdir flask_api cd flask_api # Create a virtual environment python -m venv venv # Activate the virtual environment # On Windows venv\Scripts\activate # On macOS/Linux source venv/bin/activate # Install Flask and SQLAlchemy pip install Flask SQLAlchemy

Step 2: Setting Up the Flask App

Next, let’s create a simple Flask application to serve as the foundation of our REST API.

python

from flask import Flask app = Flask(__name__)

@app.route('/') def index():

    return "Welcome to our API" 

if __name__ == '__main__':

    app.run(debug=True)

Save this in a file called app.py. To run the server, execute:

bash

python app.py

You should now be able to open a browser and navigate to http://127.0.0.1:5000/ to see the welcome message.

Step 3: Configuring SQLAlchemy

Now, let’s set up SQLAlchemy for database interactions. We’ll start by configuring the database and creating a simple model.

python

from flask_sqlalchemy import SQLAlchemy 

app = Flask(__name__) 

app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///api.db' app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False db = SQLAlchemy(app)

Let’s define a model representing a resource, such as a "Book" in a library:

python

class Book(db.Model):

    id = db.Column(db.Integer, primary_key=True)

    title = db.Column(db.String(100), nullable=False)    author = db.Column(db.String(100), nullable=False)

    def __repr__(self):

        return f'<Book {self.title}>'

Once the model is defined, create the database by running:

bash

# In Python shell 

from app import db 

db.create_all()

Step 4: Creating CRUD Routes

To make our API useful, we need to define routes for CRUD (Create, Read, Update, Delete) operations. Here’s how you can implement them:

python

from flask import request, jsonify 

# Create a new book 

@app.route('/books', methods=['POST']) 

def add_book():

    data = request.get_json()

    new_book = Book(title=data['title'], author=data['author'])

    db.session.add(new_book)

    db.session.commit()

    return jsonify({"message": "Book added"}), 201 

# Get all books 

@app.route('/books', methods=['GET']) 

def get_books():

    books = Book.query.all()

    return jsonify([{"title": book.title, "author": book.author} for book in books]) 

# Update a book 

@app.route('/books/<int:id>', methods=['PUT']) 

def update_book(id):

    book = Book.query.get(id)

    if not book:

        return jsonify({"message": "Book not found"}), 404

    data = request.get_json()

    book.title = data['title']

    book.author = data['author']

    db.session.commit()

    return jsonify({"message": "Book updated"}) 

# Delete a book 

@app.route('/books/<int:id>', methods=['DELETE']) 

def delete_book(id):

    book = Book.query.get(id)

    if not book:

        return jsonify({"message": "Book not found"}), 404

    db.session.delete(book)

    db.session.commit()

    return jsonify({"message": "Book deleted"})

Step 5: Testing the API

You can now test the API using tools like Postman or cURL. Here are some example requests:

• Add a book:

bash

curl -X POST http://127.0.0.1:5000/books \ -H "Content-Type: application/json" \ -d '{"title": "The Great Gatsby", "author": "F. Scott Fitzgerald"}'

• Get all books:

bash

curl http://127.0.0.1:5000/books

• Update a book:

bash

curl -X PUT http://127.0.0.1:5000/books/1 \ -H "Content-Type: application/json" \ -d '{"title": "The Great Gatsby", "author": "Fitzgerald"}'

• Delete a book:

bash

curl -X DELETE http://127.0.0.1:5000/books/1

Conclusion

In this guide, we covered the basic steps for setting up a REST API using Flask and SQLAlchemy. We’ve implemented the fundamental CRUD operations that serve as the backbone of many web applications. You can further extend this API by adding features like authentication, pagination, or connecting it to a more robust database like PostgreSQL.

This basic framework will allow you to build more complex applications as your needs evolve. Happy coding!