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173 lines
5.5 KiB
Markdown
173 lines
5.5 KiB
Markdown
---
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title: How to set up Python feature flags in Flask
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date: 2023-09-05
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author:
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- ian-vanagas
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showTitle: true
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sidebar: Docs
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tags:
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- feature flags
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---
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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.
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## Creating a Flask app and adding PostHog
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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.
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```bash
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mkdir flag-demo
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cd flag-demo
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touch hello.py
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```
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Next, create a virtual environment named `venv` for our app, activate the virtual environment, and install Flask.
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```bash
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python3 -m venv venv
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. venv/bin/activate
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pip install Flask
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```
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In `hello.py`, create a home route that returns a basic "Hello, World!" and a `/user/<string:user>` route that returns the user string:
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```python
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# flag-demo/hello.py
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from flask import Flask
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app = Flask(__name__)
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@app.route("/")
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def hello_world():
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return "<p>Hello, World!</p>"
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@app.route("/user/<string:user>")
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def show_user(user):
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return f"<p>Hello, {user}!</p>"
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```
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Finally run `flask --app hello run` and go to `http://127.0.0.1:5000` to see your basic app running.
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## Adding PostHog
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Next, we install and set up PostHog using the [Python SDK](/docs/libraries/python).
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```bash
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pip install posthog
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```
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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.
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```python
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# flag-demo/hello.py
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from flask import Flask
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from posthog import Posthog
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posthog = Posthog(
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'<ph_project_api_key>',
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host='<ph_client_api_host>'
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)
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app = Flask(__name__)
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@app.route("/")
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def hello_world():
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return "<p>Hello, World!</p>"
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@app.route("/user/<string:user>")
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def show_user(user):
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posthog.capture(
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"visited_user_page",
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distinct_id=user,
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properties={
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'$set_once': {'initial_name': user}
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}
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)
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return f"<p>Hello, {user}!</p>"
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```
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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.
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## Creating a feature flag
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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."
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The second release condition enables us to test the active flag state by going to the `/user/ian` route first but is optional.
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## Adding our feature flag
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With our feature flag created in PostHog, it is time to implement it in our Flask app.
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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.
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```python
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# ...
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@app.route("/user/<string:user>")
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def show_user(user):
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flag_enabled = posthog.feature_enabled(
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'new-cool-feature',
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user
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)
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posthog.capture(
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"visited_user_page",
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distinct_id=user,
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properties={
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'$set_once': {'initial_name': user}
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}
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)
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if flag_enabled:
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return f"<p>Welcome, {user}! You are on the new cool page</p>"
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return f"<p>Hello, {user}!</p>"
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```
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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.
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Lastly, we must capture the feature flag details in our event. This enables us to do usage analysis related to the flag later.
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```python
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# ...
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@app.route("/user/<string:user>")
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def show_user(user):
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flag_enabled = posthog.feature_enabled(
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'new-cool-feature',
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user
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)
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posthog.capture(
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"visited_user_page",
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distinct_id=user,
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properties={
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'$set_once': {'initial_name': user},
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'$feature/new-cool-feature': flag_enabled
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}
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)
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if flag_enabled:
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return f"<p>Welcome, {user}! You are on the new cool page</p>"
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return f"<p>Hello, {user}!</p>"
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```
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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).
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## Further reading
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- [How to set up analytics in Python and Flask](/tutorials/python-analytics)
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- [How to set up A/B tests in Python and Flask](/tutorials/python-ab-testing)
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- [How to do holdout testing](/tutorials/holdout-testing)
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