How to Create a CRUD API in Flask: Simple Guide

To create a CRUD API in Flask, define routes for each operation: create (POST), read (GET), update (PUT), and delete (DELETE). Use Flask's routing decorators and handle JSON data to build endpoints that manage resources.

Syntax

In Flask, you create API endpoints using route decorators like @app.route(). Each route corresponds to a URL and HTTP method (GET, POST, PUT, DELETE) for CRUD operations.

• POST: Create new data
• GET: Read or retrieve data
• PUT: Update existing data
• DELETE: Remove data

Use request.get_json() to get JSON data from client requests and jsonify() to send JSON responses.

python

from flask import Flask, request, jsonify

app = Flask(__name__)

# Create (POST)
@app.route('/items', methods=['POST'])
def create_item():
    data = request.get_json()
    # logic to add item
    return jsonify({'message': 'Item created', 'item': data}), 201

# Read (GET)
@app.route('/items/<int:item_id>', methods=['GET'])
def get_item(item_id):
    # logic to get item by id
    return jsonify({'item_id': item_id, 'name': 'Sample Item'})

# Update (PUT)
@app.route('/items/<int:item_id>', methods=['PUT'])
def update_item(item_id):
    data = request.get_json()
    # logic to update item
    return jsonify({'message': 'Item updated', 'item_id': item_id, 'new_data': data})

# Delete (DELETE)
@app.route('/items/<int:item_id>', methods=['DELETE'])
def delete_item(item_id):
    # logic to delete item
    return jsonify({'message': 'Item deleted', 'item_id': item_id})

Example

This example shows a simple in-memory CRUD API for managing items using Flask. It stores items in a dictionary and supports creating, reading, updating, and deleting items via HTTP requests.

python

from flask import Flask, request, jsonify

app = Flask(__name__)

items = {}
next_id = 1

@app.route('/items', methods=['POST'])
def create_item():
    global next_id
    data = request.get_json()
    item = {'id': next_id, 'name': data.get('name')}
    items[next_id] = item
    next_id += 1
    return jsonify(item), 201

@app.route('/items/<int:item_id>', methods=['GET'])
def get_item(item_id):
    item = items.get(item_id)
    if item:
        return jsonify(item)
    return jsonify({'error': 'Item not found'}), 404

@app.route('/items/<int:item_id>', methods=['PUT'])
def update_item(item_id):
    item = items.get(item_id)
    if not item:
        return jsonify({'error': 'Item not found'}), 404
    data = request.get_json()
    item['name'] = data.get('name', item['name'])
    return jsonify(item)

@app.route('/items/<int:item_id>', methods=['DELETE'])
def delete_item(item_id):
    if item_id in items:
        del items[item_id]
        return jsonify({'message': 'Item deleted'})
    return jsonify({'error': 'Item not found'}), 404

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

Output

Running Flask app on http://127.0.0.1:5000/ with endpoints:
POST /items
GET /items/<item_id>
PUT /items/<item_id>
DELETE /items/<item_id>

Common Pitfalls

• Not specifying HTTP methods in @app.route() causes routes to accept only GET by default.
• Forgetting to parse JSON with request.get_json() leads to errors when accessing data.
• Not handling missing or invalid data can cause server errors.
• Using global variables without proper care can cause concurrency issues in real apps.
• Not returning proper HTTP status codes reduces API clarity.

python

from flask import Flask, request, jsonify

app = Flask(__name__)

# Wrong: No methods specified, only GET allowed
@app.route('/items')
def create_item_wrong():
    data = request.get_json()  # Will fail on POST
    return jsonify(data)

# Right: Specify POST method
@app.route('/items', methods=['POST'])
def create_item_right():
    data = request.get_json()
    if not data or 'name' not in data:
        return jsonify({'error': 'Name is required'}), 400
    return jsonify({'message': 'Item created', 'item': data}), 201

Quick Reference

Use this quick guide for Flask CRUD API routes:

Operation	HTTP Method	Route Example	Purpose
Create	POST	/items	Add a new item
Read	GET	/items/	Get item details
Update	PUT	/items/	Modify existing item
Delete	DELETE	/items/	Remove an item

Key Takeaways

Define routes with correct HTTP methods for each CRUD operation in Flask.

Use request.get_json() to read JSON data and jsonify() to send JSON responses.

Handle missing data and errors gracefully with proper HTTP status codes.

Avoid global state for data storage in production; use databases instead.

Test each endpoint with tools like curl or Postman to verify behavior.