flask-pancake

Feature Flagging for Flask

This library was heavily inspired by django-waffle.

Installation

flask-pancake depends on Redis and the flask-redis Python package.

$ python -m pip install flask-pancake
Successfully installed flask-pancake

from flask import Flask
from flask_pancake import FlaskPancake, Switch
from flask_redis import FlaskRedis

app = Flask(__name__)
pancake = FlaskPancake(app)
redis = FlaskRedis(app)

SWITCH_FEATURE = Switch("FEATURE", default=False)


@app.route("/")
def index():
    if SWITCH_FEATURE.is_active():
        return "Hello World!", 200
    else:
        return "Not found", 404

Alternatively, if you use a create_app() method to configure your Flask app, use pancake.init_app():

from flask import Flask
from flask_pancake import FlaskPancake

pancake = FlaskPancake()


def create_app() -> Flask:
    app = Flask(__name__)
    pancake.init_app(app)
    return app

Usage

flask-pancake provides three types of flags:

• Switches, which are either globally active or inactive. A common use case for these are system-wide enabling or disabling of a feature. E.g. in the context of a dependency on a third party service, disabling a feature with a global switch when that service is unavailable.

• Flags are like Switches but can be overridden per user. To make use of Flags, one needs to define a function that returns a user's unique ID or None:

  from flask import request
  from flask_pancake import FlaskPancake
  
  def get_user_id():
      return getattr(getattr(request, "user", None), "uid", None)
  
  pancake = FlaskPancake(get_user_id_func=get_user_id)
  # Or, if importing a function from somewhere isn't possible, a string based
  # approach can be used.
  # Separate the the fully qualified module path from the function with a `:`
  pancake = FlaskPancake(get_user_id_func="my.app.account.utils:get_uid")
  

• Samples, have a global "ratio" of 0 - 100%. Each time a check is done on a sample, a random value is checked within these bounds. Hence:

  # DO THIS!
  def foo():
      is_active = MY_SAMPLE.is_active()
      if is_active:
          # do something
          pass
      ...
      if is_active:
          # do more
          pass
  
  # DO NOT DO THIS!
  def foo():
      if MY_SAMPLE.is_active():
          # do something
          pass
      ...
      if MY_SAMPLE.is_active():
          # do more
          pass
  

  In the second example, each call to is_active() will be evaluated again. Thus, the first block might be executed, but the second might not (or vice versa).

The persisted state for all three types of feature flags can be cleared, using the clear() method.

Similarly, one can change the persisted state for Flags and Switches using their disable() and enable() methods. Samples can be updated using their set(value: float) method.

When using Flags, there are clear_user() and clear_all_users() methods, to clear the state for the current or all users. Along the same line, there are disable_user() and enable_user() to set the current user's state.