Flask

What is Flask?

It's a web framework for python, packaging an HTTP server on the machine.

A Basic Flask Server

Create a python file, app.py like below.

from flask import Flask

app = Flask(__name__)

@app.route('/')
def index():
    return 'Hello World!'

if __name__ == '__main__':
    app.run(debug=True, host='0.0.0.0', port=5000)

Note the structure of app.py above. The decorator @app.route('SOMEROUTE') defines the route relative to the server's base URL. The function that follows, in this case index(), then gets called whenever its HTTP endpoint gets a request at its decorator relative route. In this case that is the root of the server. Which will then respond with an HTTP response which could have custom headers, HTML, JSON, or whatever other content is valid for an HTTP response.

Responding with HTML

To handle HTML, templating is generally recommended, but it's possible to serve static HTML files as strings as well. To do this create an HTML file index.html in the project root of the flask server, like where app.py is. Then in app.py change index to return render_template('index.html').

Debugging Flask Servers

It can get complicated debugging flask servers. For VSCode include the snippet below in the __main__ section:

if __name__ == "__main__":
    app.run(host="0.0.0.0", port=8080, use_debugger=False, use_reloader=False, passthrough_errors=True)

Then create a launch.json file in the debugger like this:

{
    "version": "0.2.0",
    "configurations": [
        {
            "name": "Run flask file",
            "type": "python",
            "request": "launch",
            "env": {
                "FLASK_APP": "app.py",
                "FLASK_ENV": "development",
                "FLASK_DEBUG": "1"
            },
            "args":[
                "run",
                "--no-debugger",
                "--no-reload"
            ],
            "jinja": true,
            "program": "${file}",
            "console": "integratedTerminal",
            "justMyCode": true
        }
    ]
}

Now it will debug the file when clicking the run-debug button, while that file is open.

Cookies in Flask

Using flask's route decorator you get request objects to inspect the request. To get cookies from the request simply use request.cookies.get('COOKIE_NAME_OR_KEY') to access it. In the below example is a book server that will either show the books if the session cookie proving a login is available, or it will go to a register page.

@app.route("/books", methods=["GET"])
def getBooks():
    try:
        user = request.cookies.get("MySessionCookie")
        return render_template('books.html', books=books, username=user)
    except:
        return render_template("register.html")

Below is a basic cookie adding, removing and retrieving; displaying and removing cookies shows the basics of what Flask can do with cookies.

from flask import Flask, request
from flask import make_response
app = Flask(__name__)

@app.route("/addCookie")
def addCookie():
    response = make_response("<h1>Cookie added!</h1>");
    # add code to add cookie here
    response.set_cookie('myFirstCookie', 'Hello World - my first cookie!')
    return response

@app.route("/displayCookieValue")
def displayCookieValue():
    cookieValue = None
    # add code to get the cookie value (inside try block)
    try:
        cookieValue = request.cookies.get('myFirstCookie')
        if cookieValue != None:
            return "<h1>Found the cookie: {}</h1>".format(cookieValue)
        else:
            return "<h1>Cookie not found!</h1>"
    except:
        return "<h1>Cookie not found!</h1>"

@app.route("/removeCookie")
def removeCookie():
    res = make_response("Cookie removed!")
    # add code to remove cookie here (set max_age = 0)
    res.set_cookie('myFirstCookie', 'Hello World - my first cookie!', max_age=0)
    return res