CS1520 Recitation:

Flask 1Jeongmin Lee

Plan for Today● Install Flask and Run First Program

● Routing

● Variable Rules

Installing Flask

Start with Virtual ENV1. Install Virtual Environment first

● pip install virtualenv

Start with Virtual ENV** For windows

1-1. Download get-pip.py file and run it

● https://bootstrap.pypa.io/get-pip.py

1-2. Setup pip and then virtual env

● pip install --upgrade pip setuptools

● pip install virtualenv

Start with Virtual ENV2. Create new environment with Python 3

● python -m virtualenv cs1520_flask -p

/Library/Frameworks/Python.framework/Versions/3.4/bin/pyt

ho

(example for my setting)

Start with Virtual ENV3. Get into the folder and activate environment

● cd/cs1520_flask

● source bin/activate

4. Now, we are in the new environment

Get Flask5. Install Flask

● pip install Flask

Note that this installation is

only on current environment.

Not your system’s Python.

Run First Application● save it as first.py

from flask import Flaskapp = Flask(__name__)

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

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

● run >> python first.py

Run First Application● save it as first.py

from flask import Flaskapp = Flask(__name__)

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

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

● run >> python first.py

Routing

RoutingRouting is about how URL and resources are linked!

● URL : Uniform Resource Locator

e.g., http://cs.pitt.edu/index.html

● Resource: can be anything. image, file, html,

even a piece of function in Python.

Routing● Important job of Flask is handling user’s request

Routing● Important job of Flask is handling user’s request

● Request is coming through URL

● Routing is a mechanism that handles URL to specific

function in your code

First Applicationfrom flask import Flaskapp = Flask(__name__)

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

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

First Applicationfrom flask import Flaskapp = Flask(__name__)

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

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

First Applicationfrom flask import Flaskapp = Flask(__name__)

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

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

● URL of ‘/’ i bound to the

hello_world() function.

● Hence, user requested ‘/’ URL,

the hello_world() function will

be run and its results will be

rendered in the browser.

Let’s have more routes!from flask import Flaskimport datetimeapp = Flask(__name__)

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

@app.route('/todayis')def get_today_date(): return str(datetime.date.today())

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

Variable Rules

Variable Rules● Key idea: Build a URL dynamically.

Variable Rules● Key idea: Build a URL dynamically.

● Use python’s variable to be the part of URL!

Variable Rules● Key idea: Build a URL dynamically.

● Use python’s variable to be the part of URL!

● Variable Rules are passed as a keyword argument to the

function with which rule is associated.

Variable Rules● Key idea: Build a URL dynamically.

● Use python’s variable to be the part of URL!

● Variable Rules are passed as a keyword argument to the

function with which rule is associated.

Variable Rules● Variable Rules are passed

as a keyword argument to

the function with which

rule is associated.

from flask import Flaskapp = Flask(__name__)

@app.route('/hello/<name>')def hello_name(name): return 'Hello %s!' % name

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

Variable Rules● You can use other type of variable.

● Integer and Floating Point numbers.

● Path (string with slash ‘/’)

Variable RulesInteger and Floating Point

● <int:postID> will cast

with coming string part of

URL into type of integer.

● Same for Float.

from flask import Flaskapp = Flask(__name__)

@app.route('/blog/<int:postID>')def show_blog(postID): return 'Blog Number %d' % postID

@app.route('/rev/<float:revNo>')def revision(revNo): return 'Revision Number %f' % revNo

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

Variable RulesPath:

● In computer world these

are different:

○ /python/ and /python

● But Flask will treat them

same for trailing slash (/)

from flask import Flaskapp = Flask(__name__)

@app.route('/flask')def hello_flask(): return 'Hello Flask'

@app.route('/python/')def hello_python(): return 'Hello Python'

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

Questions?