posthog.com/contents/tutorials/python-feature-flags.md

---
title: How to set up Python feature flags in Flask
date: 2023-09-05
author:
  - ian-vanagas
showTitle: true
sidebar: Docs
tags:
  - feature flags
---

Feature flags make it easy to conditionally run code and show content based on users or conditions. In this tutorial, we show how to create a basic Python Flask app, add PostHog, and set up [feature flags](/feature-flags) to conditionally show content in the app.

## Creating a Flask app and adding PostHog

To showcase feature flags, we use [Flask](https://flask.palletsprojects.com/), a Python web framework, to create an app. To start, we create a folder for our app named `flag-demo` and create a file named `hello.py` in that folder.

```bash
mkdir flag-demo
cd flag-demo
touch hello.py
```

Next, create a virtual environment named `venv` for our app, activate the virtual environment, and install Flask.

```bash
python3 -m venv venv
. venv/bin/activate
pip install Flask
```

In `hello.py`, create a home route that returns a basic "Hello, World!" and a `/user/<string:user>` route that returns the user string:

```python
# flag-demo/hello.py
from flask import Flask

app = Flask(__name__)

@app.route("/")
def hello_world():
  return "<p>Hello, World!</p>"

@app.route("/user/<string:user>")
def show_user(user):
    return f"<p>Hello, {user}!</p>"
```

Finally run `flask --app hello run` and go to `http://127.0.0.1:5000` to see your basic app running.

![Hello, World!](https://res.cloudinary.com/dmukukwp6/image/upload/v1710055416/posthog.com/contents/images/tutorials/python-feature-flags/hello.png)

## Adding PostHog

Next, we install and set up PostHog using the [Python SDK](/docs/libraries/python).

```bash
pip install posthog
```

We initialize PostHog to our `hello.py` file using our project API key and instance address from [your project settings](https://app.posthog.com/project/settings). In our user route, we use PostHog to capture an event using the user string and set a person property of `initial_name` on that user.

```python
# flag-demo/hello.py
from flask import Flask
from posthog import Posthog

posthog = Posthog(
  '<ph_project_api_key>',
  host='<ph_client_api_host>'
)

app = Flask(__name__)

@app.route("/")
def hello_world():
  return "<p>Hello, World!</p>"

@app.route("/user/<string:user>")
def show_user(user):
  posthog.capture(
    "visited_user_page",
    distinct_id=user,
    properties={
      '$set_once': {'initial_name': user}
    }
  )
  return f"<p>Hello, {user}!</p>"
```

When we rerun `flask --app hello run` and go to the user route such as `http://127.0.0.1:5000/user/ian`, it captures an event in PostHog.

![Event](https://res.cloudinary.com/dmukukwp6/image/upload/v1710055416/posthog.com/contents/images/tutorials/python-feature-flags/event.png)

## Creating a feature flag

We are now ready to create and set up our flag. To do this, go to the [feature flag tab](https://app.posthog.com/feature_flags) in PostHog and click "New feature flag." Add a key (we use `new-cool-feature`), set the release condition to 50% of users, and add another condition set where `initial_name` is `ian` (or whatever URL you just visited). Add any other details and press "Save."

![Feature flag](https://res.cloudinary.com/dmukukwp6/image/upload/v1710055416/posthog.com/contents/images/tutorials/python-feature-flags/flag.png)

The second release condition enables us to test the active flag state by going to the `/user/ian` route first but is optional.

## Adding our feature flag

With our feature flag created in PostHog, it is time to implement it in our Flask app.

Back in our `user` route, we add a check with PostHog of our `new-cool-feature` flag. If it is true, we return a different `<p>` tag. If it isn’t, we return the same value as before.

```python
# ...

@app.route("/user/<string:user>")
def show_user(user):
  flag_enabled = posthog.feature_enabled(
    'new-cool-feature',
    user
  )

  posthog.capture(
    "visited_user_page",
    distinct_id=user,
    properties={
      '$set_once': {'initial_name': user}
    }
  )

  if flag_enabled:
    return f"<p>Welcome, {user}! You are on the new cool page</p>"

  return f"<p>Hello, {user}!</p>"
```

Since we visited `/user/ian` first, we get the new variant while 50% of the rest of the pages get it as well. If you want to customize this, edit your feature flag to add more conditions.

![Cool new feature](https://res.cloudinary.com/dmukukwp6/image/upload/v1710055416/posthog.com/contents/images/tutorials/python-feature-flags/cool.png)

Lastly, we must capture the feature flag details in our event. This enables us to do usage analysis related to the flag later.

```python
# ...

@app.route("/user/<string:user>")
def show_user(user):
  flag_enabled = posthog.feature_enabled(
    'new-cool-feature',
    user
  )

  posthog.capture(
    "visited_user_page",
    distinct_id=user,
    properties={
      '$set_once': {'initial_name': user},
      '$feature/new-cool-feature': flag_enabled
    }
  )

  if flag_enabled:
    return f"<p>Welcome, {user}! You are on the new cool page</p>"

  return f"<p>Hello, {user}!</p>"
```

This is the basic implementation of Python feature flags in Flask set up. From here, we can set up [A/B tests](/experiments), a [public beta program](/tutorials/public-beta-program), or [canary releases](/tutorials/canary-release).

## Further reading

- [How to set up analytics in Python and Flask](/tutorials/python-analytics)
- [How to set up A/B tests in Python and Flask](/tutorials/python-ab-testing)
- [How to do holdout testing](/tutorials/holdout-testing)

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