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How to set up Python analytics in Flask

Feb 22, 2024

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Product analytics enable you to gather and analyze data about how users interact with your Python app. To show you how to set up analytics, in this tutorial we create a basic Python app with Flask, add PostHog, and use it to capture events and create insights.

1. Create a basic Flask app

We start by creating a simple Flask app that has two pages:

  1. A login page where a user can enter their name, email, and company name in a form.
  2. A dashboard page that has some text and a button.

First, make sure Python and Flask are installed. If Flask is not installed, you can install it via pip:

Terminal
pip3 install Flask

Create a new folder for your project called flask-analytics and set up a basic project structure:

Terminal
mkdir flask-analytics
cd flask-analytics
mkdir templates static
touch app.py
cd ./templates
touch login.html dashboard.html
cd ..

Then, we set up our app routes in app.py:

app.py
from flask import Flask, render_template, request, redirect, session, url_for


app = Flask(__name__)
app.secret_key = 'a_super_secret_key!' # you don't need to replace this


@app.route('/')
def index():
    return render_template('login.html')


@app.route('/login', methods=['POST'])
def login():
    session['name'] = request.form['name']
    session['email'] = request.form['email']
    session['company_name'] = request.form['company_name']
    return redirect(url_for('dashboard'))


@app.route('/dashboard')
def dashboard():
    if 'name' in session:
        return render_template('dashboard.html', session=session)
    return redirect(url_for('index'))


@app.route('/api/dashboard', methods=['POST'])
def api_dashboard():
    # Placeholder for API logic. We'll add this later
    return '', 204


if __name__ == '__main__':
    app.run(port=8000, debug=True)

Lastly, we set up the basic layout for our HTML pages:

templates/login.html
<!DOCTYPE html>
<html>
<head>
    <title>Login</title>
    <style>
        label, input, button {
            display: block;
            margin-bottom: 10px;
        }
    </style>
</head>
<body>
    <form action="{{ url_for('login') }}" method="POST">
        <label for="name">Name:</label>
        <input type="text" name="name" required>


        <label for="email">Email:</label>
        <input type="email" name="email" required>


        <label for="company_name">Company Name:</label>
        <input type="text" name="company_name" required>


        <button type="submit">Log in</button>
    </form>
</body>
</html>
templates/dashboard.html
<!DOCTYPE html>
<html>
<head>
    <title>Dashboard</title>
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.6.0/jquery.min.js"></script>
</head>
<body>
    <h1>Welcome, {{ session['name'] }} from {{ session['company_name'] }}!</h1>


    <form id="dashboardForm" action="{{ url_for('api_dashboard') }}" method="POST">
        <input type="hidden" name="email" value="{{ session['email'] }}">
        <input type="hidden" name="name" value="{{ session['name'] }}">
        <input type="hidden" name="company_name" value="{{ session['company_name'] }}">
        <button type="submit">Click Me</button>
    </form>


    <script>
    $(document).ready(function() {
        $('#dashboardForm').on('submit', function(e) {
            e.preventDefault();
            $.ajax({
                url: $(this).attr('action'),
                type: 'POST',
                data: $(this).serialize(),
            });
        });
    });
    </script>
</body>
</html>

Run python3 app.py and navigate to http://localhost:8000 to see our app in action. Enter anything on the login page to save some session details.

2. Add PostHog to your app

With our app set up, it’s time to install and set up PostHog. If you don't have a PostHog instance, you can sign up for free.

Run pip3 install posthog to install PostHog's Python SDK. Then, initialize PostHog in app.py using your project API key and instance address (you can find these in your project settings):

app.py
package main
from flask import Flask, render_template, request, redirect, session, url_for
from posthog import Posthog


posthog = Posthog(
  '<ph_project_api_key>', 
  host=https://us.i.posthog.com
)


# rest of your code

With this set up, we're ready to capture events.

3. Implement the event capture code

To show how to capture events with PostHog, we capture an event when the button on the dashboard page is clicked. To do this, we call posthog.capture():

app.py
@app.route('/api/dashboard', methods=['POST'])
def api_dashboard():
    email = request.form.get('email')
    posthog.capture(
        email, 
        'home_api_called'
    )
    return '', 204

With this set up, refresh your app and click the button on the dashboard page a few times. You should now see the captured event in your PostHog activity tab.

Events in PostHog

💡 PostHog tip: Setting the correct distinct_id

When calling posthog.capture(), you need to provide a distinct_id argument. This is a unique identifier for your user and enables you to correctly attribute events to them.

For logged-in users, you typically use their email or database ID. For logged-out or anonymous users, you should use a unique identifier, either generated by you or the PostHog JavaScript web library (which can then be accessed in the cookies. See an example of accessing the PostHog cookie in our Nuxt tutorial).

Setting event properties

When capturing events, you can optionally include additional information by setting the properties argument. This is helpful for breaking down or filtering events when creating insights.

As an example, we add the user's name as an event property:

app.py
@app.route('/api/dashboard', methods=['POST'])
def api_dashboard():
    email = request.form.get('email')
    name = request.form.get('name')
    posthog.capture(
        email, 
        'home_api_called',
        properties={
            "user_name": name
        }
    )
    return '', 204

Capturing group events

Groups are a powerful feature in PostHog that aggregate events based on entities, such as organizations or companies. This is especially helpful for B2B SaaS apps, where often you want to view insights such as number of active companies or company churn rate.

To enable group analytics, you'll need to upgrade your PostHog account to include them. This requires entering your credit card, but don't worry, we have a generous free tier of 1 million events per month – so you won't be charged anything yet.

To create groups in PostHog, simply include them in your code when capturing events by setting the groups argument:

app.py
@app.route('/api/dashboard', methods=['POST'])
def api_dashboard():
    email = request.form.get('email')
    name = request.form.get('name')
    company_name = request.form.get('company_name')
    posthog.capture(
        email, 
        'home_api_called',
        properties={
            "user_name": name
        },
        groups={'company': company_name}
    )
    return '', 204

In the above example, we create a group type company, and then set the value as the unique identifier for that specific company. This enables us to breakdown insights by company (we show you how to do this in the next section).

4. Create an insight in PostHog

Restart your app and capture events using different inputs in the login page. This will capture events for different users and companies and enable us to show the power of PostHog insights.

Next, go to the Product analytics tab in PostHog and click the + New insight button. PostHog supports many different types of insights, such as trends, funnels, paths and more.

In this tutorial, we create a simple trend insight:

  1. Select the Trends tab.
  2. Under the Series header select the home_api_called event.
  3. Click the Total count dropdown to change how events are aggregated. You can choose options such as Count per user, Unique users, Unique company(s), and more. You can also add filters or breakdown based on properties.

For example, in the image below we set our insight to show number of unique users that captured the home_api_called event where the user_name property is equal to Max:

Insight created in PostHog

That's it! Feel free to play around in your dashboard and explore the different kinds of insights you can create in PostHog.

Further reading