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Flask Sessions

Posted by Julian on Fri 02 June 2017 in Flask • 4 min read

More Flask?! Yes! More Flask! I apologise for nothing! This is however a quick article for anyone wanting to learn about Flask Session Objects.

One of my most exciting discoveries as of late has been the session object. I stumbled upon this useful little thing while making my Pay Calculator App for our 100 Days of Code Challenge.

Let’s discuss sessions in the usual Julian format.

What is a Flask Session Object?

Think of a Flask Session Object as a special variable that persists for the life of the browser session that’s connected to the Flask app.

Say What?!

Well, here:

I wanted to make a pay calculator web application that allowed me to calculate how much money I’d be paid based on me entering my hourly wage and how many hours I worked. Simple right?

Well, what if, as part of this app, I want to have another web page that could extrapolate my entire annual salary from just the hourly wage? I don’t want these two functions to exist on the same page.

I also don’t want to ask the user to enter in their salary every time they flick between these two pages/apps, that’d be annoying right? I’d need a variable that could store the hourly wage and keep that data accessible to any of the web apps running from this Flask app.

This is where you’d use a session object. You’d assign the user’s hourly wage to the session object which you would then make available to use across different web pages in your app.

Code Me Up!

The thing that surprised me was how simple it was. Coding wise, you use the object in exactly the same way you’d use a normal variable. It still has to abide by normal global/local rules in your code too.

To assign the hourly wage to a session object I did this:

session['wage'] = float(request.form.get('wage'))

The code on the right is just pulling in the data from the HTML form with the name “wage”. It stores that as a float into a session object of the same name (wage).

It’s seriously as simple as that.

Super Secret Keys

I know all of this sounds a lot like cookies. It should. Flask session objects exist on top of standard cookies. All that’s different is that the cookie is locked down with a secret key.

This does not mean the data is private! The cookie data is visible but cannot be modified unless you have the secret key.

This secret key needs to be accessible to your app code. You’d preferably make it an environment variable that isn’t accessible to the outside world. I made mine as complex as possible:

app.secret_key = "Test_Secret_Key"

Test_Secret_Key being the password. Good luck cracking this bad boy!

The Pay Calc App Use Case

In my Pay Calculator App, I create the wage session object in my index ‘/‘ route:

@app.route('/', methods=['GET', 'POST'])
def index():    
    if request.method == 'POST' and 'wage' in request.form:
        session['wage'] = float(request.form.get('wage'))
        return redirect(url_for('pay_calc'))
    return render_template("index.html")

What you’ll notice is that wage is created and then returned in the redirect to the page associated with the pay_calc function.

This essentially makes it available to the second page of the app.

I then check for it in the code for the second page:

@app.route('/pay', methods=['GET', 'POST'])
def pay_calc():
    pay = ''
    if request.method == 'POST' and 'hours' in request.form and 'wage' in session:
        hours = float(request.form.get('hours'))
        pay = calc_wage(session['wage'], hours)
    return render_template("pay_calc.html",

The if statement checks to see if the wage session object exists. If it doesn’t, the calculation within the if won’t take place.

Other Use Cases

  • You could use this for a personal touch on your site. A user enters their name which allows you to refer to them by their name on relevant screens.

  • A to-do list! The entire list is in the session object which can be called and loaded on any page the user loads on your site. It’d be a good idea to have data persistence as well in this scenario though!

  • Dare I say it? Ordering food online is a great use case for this. Add all selected items to a session object to keep track of the user order while they browse. Once the order is complete, the data can be scrapped when the browser session closes.

  • Any sort of online calculation service: currency exchange, electricity, insurance, superannuation. These all require temporary data.

  • Flight tracking information (I may be stretching it here). A user would enter details of a flight to track and that data would stay live for the duration of the session. There’s no need for it to be stored.

  • Even an old school type of browser game like Jetman. The score is retained for the duration of the session but once the browser closes, you start from scratch.


I think you get the point! The session object is incredibly useful!

I’ll admit, it did take some time to wrap my head around how to get it working at a code level. The concept is simple but writing the code such that it worked took some testing and playing.

If you have any cool use cases or examples of session object usage, let us know!

And as always, Keep Calm and Code in Python!

-- Julian

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