How to Create REST API in Flask: Simple Guide with Example

To create a REST API in Flask, define routes using the @app.route decorator and handle HTTP methods like GET and POST in your functions. Use Flask to create the app and return JSON responses with jsonify.

Syntax

In Flask, you create REST API endpoints by defining routes with the @app.route decorator. You specify the URL path and allowed HTTP methods. Inside the function, you handle the request and return a response, often in JSON format.

• @app.route('/path', methods=['GET', 'POST']): Defines the URL and HTTP methods.
• def function_name(): The function that runs when the route is accessed.
• return jsonify(data): Sends JSON response back to the client.

python

from flask import Flask, jsonify, request

app = Flask(__name__)

@app.route('/resource', methods=['GET', 'POST'])
def resource():
    if request.method == 'GET':
        return jsonify({'message': 'This is a GET request'})
    elif request.method == 'POST':
        data = request.get_json()
        return jsonify({'you_sent': data}), 201

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

Example

This example shows a simple Flask REST API with one endpoint /items. It supports GET to list items and POST to add a new item. The API returns JSON responses.

python

from flask import Flask, jsonify, request

app = Flask(__name__)

items = []

@app.route('/items', methods=['GET', 'POST'])
def items_api():
    if request.method == 'GET':
        return jsonify({'items': items})
    elif request.method == 'POST':
        item = request.get_json().get('item')
        if item:
            items.append(item)
            return jsonify({'message': 'Item added', 'items': items}), 201
        else:
            return jsonify({'error': 'No item provided'}), 400

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

Output

 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
 * Restarting with stat
 * Debugger is active!

Common Pitfalls

Common mistakes when creating REST APIs in Flask include:

• Not specifying HTTP methods in @app.route, so only GET works by default.
• Forgetting to parse JSON from request.get_json() for POST requests.
• Returning plain strings instead of JSON responses, which can confuse clients.
• Not setting proper HTTP status codes like 201 for resource creation or 400 for bad requests.

Example of a wrong and right way:

python

from flask import Flask, jsonify, request

app = Flask(__name__)

# Wrong: No methods specified, only GET allowed
@app.route('/wrong')
def wrong():
    return 'This only supports GET'

# Right: Methods specified and JSON response
@app.route('/right', methods=['POST'])
def right():
    data = request.get_json()
    if not data:
        return jsonify({'error': 'Missing JSON'}), 400
    return jsonify({'received': data}), 200

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

Quick Reference

Tips for creating REST APIs in Flask:

• Always specify methods=['GET', 'POST', ...] in routes.
• Use request.get_json() to get JSON data from clients.
• Return responses with jsonify() for proper JSON formatting.
• Set HTTP status codes to indicate success or errors.
• Run Flask with debug=True during development for helpful error messages.

Key Takeaways

Use @app.route with methods to define REST API endpoints in Flask.

Handle JSON input with request.get_json() and respond with jsonify().

Always set appropriate HTTP status codes for clarity.

Test your API endpoints with tools like curl or Postman.

Run Flask in debug mode during development for easier troubleshooting.