Cheat Sheet: Web App Deployment Using Flask

Quick reference for building simple Flask endpoints and handling HTTP status codes.

Estimated time needed: 5 minutes

Flask

Used to instantiate an object of the Flask class named app.

from flask import Flask

app = Flask(__name__)

@app.route decorator

A decorator in Flask used to map URLs to specific functions in a Flask application.

from flask import Flask

app = Flask(__name__)

@app.route('/')
def hello_world():
    return "My first Flask application in action!"

200 OK status

Flask returns a 200 OK status by default when a route returns a value. You can also return it explicitly.

from flask import Flask

app = Flask(__name__)

@app.route('/')
def hello_world():
    # Implicit 200 OK
    return "My first Flask application in action!"

@app.route('/explicit')
def explicit_ok():
    # Explicit 200 OK
    return "My first Flask application in action!", 200

Error statuses

• 400: Invalid request (e.g., missing or improper parameters).
• 401: Missing or invalid credentials.
• 403: Insufficient permissions to fulfill the request.
• 404: Resource not found.
• 405: Method not allowed.

Example returning validation and not-found errors:

from flask import Flask, request

app = Flask(__name__)

@app.route('/search')
def search_response():
    query = request.args.get("q")
    if not query:
        return {"error_message": "Input parameter missing"}, 422
    # fetch the resource from the database
    resource = fetch_from_database(query)  # implement this function
    if resource:
        return {"message": resource}
    else:
        return {"error_message": "Resource not found"}, 404

Error 500

500 is used when there is an error on the server.

from flask import Flask

app = Flask(__name__)

@app.errorhandler(500)
def server_error(error):
    return {"message": "Something went wrong on the server"}, 500

Author(s)

Andrew Pfeiffer

Other Contributor(s)

Abhishek Gagneja,