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Short introduction guide on Flask. http://flask.pocoo.org/ (Used for XPPL, so to see a more advanced use in connection with database, see XPPL)

Basic Flask

Flask is a microframework for python to create web applications. It basically connects the webserver with your python code.


$ pip install Flask

Simple text serving

from flask import Flask
app = Flask(__name__)

def hello():
    return "Hello World!"
  • with @app.route you can define the url flask respons to.
  • the function definition after is mandetory as well as the return.
  • everything that comes after return gets sent back to the browser (http GET request)

Run it with:

$ FLASK_APP=hello.py flask run


You can use any route you like


You can also use variable routes (example for int)

def book(id):

as you can see you can grab the variable in the url through the function’s parameter

(example for string)

def book(bookname):

Http Methods

Methods like GET or POST (DELETE, PUT…) can be handled by flask

Therefore you need to add the wanted methods to the route definition like:

from flask import Flask
app = Flask(__name__)

@app.route('/address_to_post', methods= ['POST','GET'])
def respond_to_post():
    answer = ""
    if request.method == 'GET':
        answer = "get"
    if request.method == 'POST':
        answer = "get"
    return answer

with request.method you can determine the incoming kind of request.


To be able return full html pages, flask uses templates using Jinja to insert variable content.

def home():
    message = "Welcome Home!"
    return render_template('home.html', message=message)

you can pass as many variables to the template as you want. in this example we pass message to the template:

The html looks something like:


!important: The html file needs to be saved inside a templates folder called "templates" inside your project folder-

Helpful function

404 Page not found

def page_not_found(error):
    """Custom 404 page."""
    return render_template('404.html'), 404