Flask FeatureFlags
This is a Flask extension that adds feature flagging to your applications. This lets you turn parts of your site on or off based on configuration.
It's useful for any setup where you deploy from trunk but want to hide unfinished features from your users, such as continuous integration builds.
You can also extend it to do simple a/b testing or whitelisting.
Installation
Installation is easy with pip:
pip install flask_featureflags
To install from source, download the source code, then run this:
python setup.py install
Flask-FeatureFlags supports Python 2.6, 2.7, and 3.3+ with experimental support for PyPy.
Version 0.1 of Flask-FeatureFlags supports Python 2.5 (but not Python 3), so use that version if you need it. Be aware that both Flask and Jinja have dropped support for Python 2.5.
Docs
For the most complete and up-to-date documentation, please see: https://flask-featureflags.readthedocs.org/en/latest/
Setup
Adding the extension is simple:
from flask import Flask
from flask_featureflags import FeatureFlag
app = Flask(__name__)
feature_flags = FeatureFlag(app)
In your Flask app.config, create a FEATURE_FLAGS dictionary, and add any features you want as keys. Any UTF-8 string is a valid feature name.
For example, to have 'unfinished_feature' hidden in production but active in development:
class ProductionConfig(Config):
FEATURE_FLAGS = {
'unfinished_feature' : False,
}
class DevelopmentConfig(Config):
FEATURE_FLAGS = {
'unfinished_feature' : True,
}
Note: If a feature flag is used in code but not defined in FEATURE_FLAGS, it's assumed to be off. Beware of typos.
If you want your app to throw an exception in dev when a feature flag is used in code but not defined, add this to your configuration:
RAISE_ERROR_ON_MISSING_FEATURES = True
If app.debug=True, this will throw a KeyError instead of silently ignoring the error.
Usage
Controllers/Views
If you want to protect an entire view:
from flask import Flask
import flask_featureflags as feature
@feature.is_active_feature('unfinished_feature', redirect_to='/old/url')
def index():
# unfinished view code here
The redirect_to parameter is optional. If you don't specify, the url will return a 404.
If your needs are more complicated, you can check inside the view:
from flask import Flask
import flask_featureflags as feature
def index():
if feature.is_active('unfinished_feature') and some_other_condition():
# do new stuff
else:
# do old stuff
Templates
You can also check for features in Jinja template code:
{% if 'unfinished_feature' is active_feature %}
new behavior here!
{% else %}
old behavior...
{% endif %}
Using other backends
Want to store your flags somewhere other than the config file? There are third-party contrib modules for other backends.
Please see the documentation here: https://flask-featureflags.readthedocs.org/en/latest/contrib.html
Feel free to add your own - see CONTRIBUTING.rst for help.
Customization
If you need custom behavior, you can write your own feature flag handler.
A feature flag handler is simply a function that takes the feature name as input, and returns True (the feature is on) or False (the feature is off).
For example, if you want to enable features on Tuesdays:
from datetime import date
def is_it_tuesday(feature):
return date.today().weekday() == 2:
You can register the handler like so:
from flask import Flask
from flask_featureflags import FeatureFlag
app = Flask(__name__)
feature_flags = FeatureFlag(app)
feature_flags.add_handler(is_it_tuesday)
If you want to remove a handler for any reason, simply do:
feature_flags.remove_handler(is_it_tuesday)
If you try to remove a handler that was never added, the code will silently ignore you.
To clear all handlers (thus effectively turning all features off):
feature_flags.clear_handlers()
Clearing handlers is also useful when you want to remove the built-in behavior of checking the FEATURE_FLAGS dictionary.
To enable all features on Tuesdays, no matter what the FEATURE_FLAGS setting says:
from flask import Flask
from flask_featureflags import FeatureFlag
app = Flask(__name__)
feature_flags = FeatureFlag(app)
feature_flags.clear_handlers()
feature_flags.add_handler(is_it_tuesday)
Chaining multiple handlers
You can define multiple handlers. If any of them return true, the feature is considered on.
For example, if you want features to be enabled on Tuesdays or Fridays:
feature_flags.add_handler(is_it_tuesday)
feature_flags.add_handler(is_it_friday)
Important: the order of handlers matters! The first handler to return True stops the chain. So given the above example, if it's Tuesday, is_it_tuesday will return True and is_it_friday will not run.
You can override this behavior by raising the StopCheckingFeatureFlags exception in your custom handler:
from flask_featureflags import StopCheckingFeatureFlags
def run_only_on_tuesdays(feature):
if date.today().weekday() == 2:
return True
else:
raise StopCheckingFeatureFlags
If it isn't Tuesday, this will cause the chain to return False and any other handlers won't run.
Acknowledgements
A big thank you to LinkedIn for letting me opensource this, and for my coworkers for all their feedback on this project. You guys are great. :)
Questions?
Feel free to ping me on twitter @trustrachel or on the Github project page.
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