Flask

Flask is a micro webdevelopment framework for Python.

• What is Web Server?

The application responsible for sending HTML to browsers is called a Web Server.

Browsers send http request (get, post, put, delete) to Web Server.

And Web Server reply with a http response, usually containing HTML that represents the page requested.

• Route

Decorator route is used to bind a function a URL:

@app.route('/')
def index():
    ...

By default, a route only answers Get requests, but that can be changed by providing methods argument to the route() decorator.

@app.route('/', methods=['POST', 'GET'])
def index():
    if request.method == 'POST':
        ...
    else:
        ...

• Static Files

Create a folder named static, and store CSS and JavaScript files in this folder.

• Rendering Template

Create a folder named templates, and store all the templates in this folder.

So, the filesystem looks like this:

/weather_report_web0.py
/templates
    index.html
/static
    styles.css

Then use render_template() to render template.

from flask import request
from flask import render_template


@app.route('/', methods=['POST', 'GET'])
def index():
    if request.method == 'POST':
        render_template('weather.html', weather=result)
    else:
        render_template('index.html')

• The Request Object

  1. Import request
  2. Use request.method to get current request method
  3. Use request.form[] to access request data in POST method
  4. Use request.args.get() to access parameters submitted in URL with GET method

from flask import request
from flask import render_template


@app.route('/', methods=['POST', 'GET'])
def index():
    if request.method == 'POST':
        location = request.form['location'].strip()
        unit = request.form['unit']
        if request.form['submit'] == 'Search':
            result = ...
            return render_template('weather.html', weather=result)
        elif request.form['submit'] == 'History':
            result = ...
            return render_template('history.html', history=result)
    else:
        submit = request.args.get('submit', '')
        if submit == 'Help':
            result = ...
            return render_template('help.html', help=result)
        else:
            return render_template('index.html')

• Run Flask app

For Windows

setx FLASK_APP weather_report_web0.py
flask run
    * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)

The server is only accessible from your own computer, if you have the debugger mode disabled or trust the users on your network, then you can make it publicly available like this:

flask run --host=0.0.0.0
    * Running on http://0.0.0.0:5000/ (Press CTRL+C to quit)

• Debug Mode

setx FLASK_DEBUG 1
flask run

• Run as Python script

Or add app.run(), then just run the py script

app = Flask(__name__)

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

> py weather-report-web0.py
Running on http://127.0.0.1:5000/ (Press CTRL+C to quit
Restarting with stat
Debugger is active!
Debugger pin code: 112-163-277