23.5. Try It! Flask Sessions

Let’s build a simple Flask application that uses sessions to track a list of items. It doesn’t matter what the list is (movies, groceries, places to visit, etc.). The key is to practice saving and accessing persistent data.


For the program to work properly, cookies must be enabled in the browser.

23.5.1. Clone the Repository

The starter code for this demo is saved on GitHub. Use this link to navigate to the repository, then follow these setup steps:

  1. On the repository page, click the green Code button and copy the URL.

    The clone dialog box with copy button highlighted.
  2. Launch Visual Studio Code. From the File menu, open your local_practice directory.

  3. In the terminal pane, enter the command:

    $ git clone repo_URL

    Be sure to replace repo_URL with the web address you copied in step 1.

  4. From the File menu, open the LCHS_session_try_it folder.

  5. Create a new virtual environment and install Flask.

    $ python3 -m venv session-env
    $ . session-env/bin/activate
    (session-env) $ pip3 install Flask
    $ py -3 -m venv session-env
    $ . session-env/Scripts/activate
    (session-env) $ pip install Flask
  6. Launch the main.py program and open the webpage in the browser. Nothing special happens on the page yet. You just need to make sure the starter code runs.

    Form page displayed when the Flask app runs the first time.

    The starter code displays a simple form.

  7. Before moving on, be sure to save and commit your work.

23.5.2. Save Session Data

In this section, you’ll use the session object to store a piece of data.

Right now, all the Flask application does is render the index.html page. Opening main.py, we see very little code inside the central if/else block.


The index() function in main.py:

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

pass is another example of a placeholder. The keyword fills the space where Python expects a block of statements. It allows you to run the starter code successfully and worry about filling in specific commands later.

Since your program will track a collection of items, start by saving an empty list to the session.

  1. To set a new session cookie, the general syntax is:

    session['key'] = value

    Just like a dictionary, key should be a string or a string variable. However, value can be any Python data type.

  2. In main.py, replace the pass keyword on line 12 with the statement:

    session['list_name'] = []

    Be sure to use something more descriptive than list_name. For example, movies, places_to_visit, groceries, etc.

  3. Save your changes. If main.py is not currently running, start it again.

  4. On the webpage, use the browser tools to confirm that a session cookie was set.

Congratulations! You just made the browser store a file on your device. It only contains an empty list, but you’ll soon fix that.


Remember, unlike plain cookies, session values can be non-string data types!

23.5.3. Access Session Data

OK, you’ve saved a session file to your device. Next you’ll learn how to access that data when you need it.

To access session data, the general syntax is:

all_session_data = session

# OR

specific_session_value = session['key']

When placed to the right of the = operator, session returns all of its key/value pairs. You can use all_session_data just like a Python dictionary.

session['key'] returns the value assigned to key. Since the session object can store multiple key/value paris, this syntax is helpful when you only want one specific entry.

Now put this to use:

  1. Open index.html in Visual Studio Code.

  2. Just beneath the form code, there is a section that displays the contents of the list.

    <section class="centered">
       <h2>List Items:</h2>
       <p>Nothing here yet...</p>
  3. On line 23, replace Nothing here yet... with a placeholder:

    <section class="centered">
       <h2>List Items:</h2>

    {{session['list_name']}} returns the value assigned to list_name.

  4. Save, then reload the page. You should see a set of empty list brackets under the form.

    Empty list brackets appear below the List Items heading.
  5. Return to main.py. Instead of assigning the empty list to session['list_name'], try assigning a list that contains one or more items. Save, then reload the page. You should see the items appear.

    A list with items appears below the Grocery List heading.

Notice that the render_template function does NOT include any variables. Since the session data is stored on your device, it is accessible by both main.py and index.html.


At any time, you can delete a session cookie using the browser tools. Open up the storage tab and right click on the item you want to remove.

Showing storage panel with menu options to delete a session cookie. Update the List Display

To make the webpage look better, let’s update the HTML code to display the elements in an unordered list.

<section class="centered">
   <h2>List Items:</h2>
      {% for item in session['list_name'] %}
      {% endfor %}

The style.css page contains some CSS rules to align and size the items, but you may need to adjust these to suit your screen.

23.5.4. Change Session Data

After learning how to create and access a session cookie, the next step is to learn how to change session data. That will be the goal for the next page.

Before moving on, take a moment to save and commit your work.

23.5.5. Check Your Understanding


To add a new key/value pair to a session object, the syntax is:

  1. session = {key : value}
  2. session['key'] = value
  3. value = session['key']
  4. session['key' : value]


Which of the following will print ONE value from the session object?

  1. print(session('key'))
  2. print(session)
  3. print(session['key'])
  4. print(session[0])