Flask uses the MVC (Model View Controller) Framework. Just to make this as obvious as possible, I like my Flask apps to have the following.

• Model -> models.py
• View -> views.py
• Controller -> __init__.py

Let's say your Flask app has the following structure.

├── main.py
├── database (directory)
│   ├── example.db
├── my-project (directory)
│   ├── __init__.py
│   ├── views.py
│   ├── models.py
│   ├── templates (directory)
│   │   ├── base.html
│   │   ├── home.html
│   │   ├── results.html
│   └── static (directory)
│       └── custom.css

In your controller (__init__,py in this example), set a SECRET_KEY, which is used to "salt" a hashed password.

app.config['SECRET_KEY'] = 'some_random_string_of_data'

In your view (views.py in this example), import session, and create a session key and value.

from flask import render_template, session

@views.route('/')
def home():    
    session["foo"] = "bar"
    return render_template('home.html')

In this example, the home.html template is rendered. Here is how you could use the session value in HTML.

{% extends "base.html" %}
{% block content %}
{% if session.foo %}
  session.foo contains a value of {{ session.foo }}
{% else %}
  session.foo does not exist
{% endif %}
{% endblock %}

And here is how you can do something based on the value of a session key in Python.

@views.route('/test')
def test():    

    try:
      session.get('foo')
    except KeyError:
      return "session.get('foo') raised KeyError"
    else:
      print("session foo = " + str(session.get('foo'))

      if session.get('foo') == 'bar':
        return "session foo equals bar"
      else:
        return "session food does not equal bar"

Or, as a bit of a more practical example, here is how you could create a session when a user signs in, and clear sessions when a user signs out.

from flask import render_template, session

@views.route('/signin')
def signin():    
    session["signedin"] = True
    return render_template('home.html')

@views.route('/signout')
def signout():    
    session.clear()
    return render_template('home.html')