install

~/Programming/Projects/Python$ mkdir Flask
~/Programming/Projects/Python$ cd Flask/
~/Programming/Projects/Python/Flask$ virtualenv app
~/Programming/Projects/Python/Flask$. app/bin/activate
~/Programming/Projects/Python/Flask$ cd app/
~/Programming/Projects/Python/Flask/app$ cd ..
~/Programming/Projects/Python/Flask$ vim hello.py
~/Programming/Projects/Python/Flask$ cat hello.py 
from flask import Flask
app = Flask(__name__)

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

if __name__ == '__main__':
    app.run(host="0.0.0.0")
~/Programming/Projects/Python/Flask$ python hello.py