DEVELOPING FLASK EXTENSIONS

Rachel Sanders @ PyTennessee 2014

Rachel Sanders Engineer at LinkedIn & PyLadiesSF

organizer Our internal stack: Python + Flask Team lead for a 30K LOC Flask app

What we talkin’ about The really really really quick intro to

Flask The really really quick intro to extending

Flask Flask-FeatureFlags: a case study Beyond the Basics Questions

Like 3 min tops, promise

The really really quick intro to Flask

from flask import Flask

app = Flask(__name__)

@app.route("/")

def hello():

return "Hello San Dimas High!"

if __name__ == "__main__":

app.run()

whaaaaaaa? No database?? No forms? No admin interface?

“The idea of Flask is to build a good foundation for all

applications. Everything else is up to you or

extensions.” -- Armin Ronacher, creator of

Flask

All these are extensions database SQLAlchemy + Flask-

SQLAlchemy forms WTForms + FlaskWTF admin Flask-Admin

Like 5 min, promise

The really quick intro to extending Flask

from flask import Flask

app = Flask(__name__)

@app.route("/")

def hello():

return "Hello San Dimas High!"

if __name__ == "__main__":

app.run()

from flask import Flask

app = Flask(__name__)

@app.route("/")

def hello():

return "Hello San Dimas High!"

if __name__ == "__main__":

app.run()

extending Flask

= changing app

You can change the app object by

Hooking into request lifecycle Adding more resources

Jinja filters, tests, global variables Routes, blueprints, static files

Middleware Monkeypatching

with the power to turn code on or off

Flask-FeatureFlags

from flask import Flask

app = Flask(__name__)

@app.route("/")

def hello():

return "Hello San Dimas High!”

from flask import Flask

import flask_featureflags as feature

app = Flask(__name__)

feature_flag = feature.FeatureFlag(app)

@app.route("/")

def hello():

return "Hello San Dimas High!"

from flask import Flask

import flask_featureflags as feature

app = Flask(__name__)

feature_flag = feature.FeatureFlag(app)

@app.route("/")

def hello():

if feature.is_active('pytennessee'):

return "Hello PyTennessee!"

else:

return "Hello San Dimas High!"

from flask import Flask

import flask_featureflags as feature

app = Flask(__name__)

feature_flag = feature.FeatureFlag(app)

app.config['FEATURE_FLAGS']['pytennessee'] = True

@app.route("/")

def hello():

if feature.is_active('pytennessee'):

return "Hello PyTennessee!"

else:

return "Hello San Dimas High!"

{% if 'pytennesse' is active_feature %} “Hi PyTennessee!”{% else %} “Hello San Dimas High!”{% endif %}

FEATURE FLAG SPEC

feature flags stored in config Jinja template test called “active_feature” Module function called “is_active”

bit.ly/giantwallofcode

class FeatureFlags(object):

class FeatureFlags(object):

def __init__(self, app=None):

class FeatureFlags(object):

def __init__(self, app=None): if app is not None: self.init_app(app)

def init_app(self, app):

class FeatureFlags(object):

def __init__(self, app=None): if app is not None: self.init_app(app)

def init_app(self, app):

app.config.setdefault('FEATURE_FLAGS’, {})

from flask import current_app

class FeatureFlags(object):

def __init__(self, app=None): if app is not None: self.init_app(app)

def init_app(self, app):

app.config.setdefault('FEATURE_FLAGS’, {})

def in_config(self, feature): try: return current_app.config['FEATURE_FLAGS'][feature] except (AttributeError, KeyError): return False

FEATURE FLAGS SPEC

feature flags stored in config Jinja template test is_active function

{% if 'pytennesse' is active_feature %} “Hi PyTennessee!”{% else %} “Hello San Dimas High!”{% endif %}

from flask import current_appclass FeatureFlags(object):

def __init__(self, app=None): if app is not None: self.init_app(app)

def init_app(self, app):

app.config.setdefault('FEATURE_FLAGS’, {})

app.add_template_test(self.in_config, name=’active_feature’)

def in_config(self, feature): try: return current_app.config['FEATURE_FLAGS'][feature] except (AttributeError, KeyError): return False

class FeatureFlags(object):

def __init__(self, app=None): if app is not None: self.init_app(app)

def init_app(self, app):

app.config.setdefault('FEATURE_FLAGS’, {})

if hasattr(app, "add_template_test"): app.add_template_test(self.in_config, name='active_feature') else: app.jinja_env.tests[’active_feature’] = self.in_config

FEATURE FLAGS SPEC

feature flags stored in config Jinja template test is_active function

from flask import Flask

import flask_featureflags as feature

app = Flask(__name__)

feature_flag = feature.FeatureFlag(app)

app.config['FEATURE_FLAGS']['pytennessee'] = True

@app.route("/")

def hello():

if feature.is_active('pytennessee'):

return "Hello PyTennessee!"

else:

return "Hello San Dimas High!"

class FeatureFlags(object):

def __init__(self, app=None): if app is not None: self.init_app(app)

def init_app(self, app):

app.config.setdefault('FEATURE_FLAGS’, {})

if hasattr(app, "add_template_test"): app.add_template_test(self.in_config, name='active_feature') else: app.jinja_env.tests[’active_feature’] = self.in_config

app.extensions['FeatureFlags'] = self

from flask import current_app

def is_active(feature): feature_flagger = current_app.extensions['FeatureFlags'] return feature_flagger.in_config(feature)

FEATURE FLAGS SPEC

feature flags stored in config Jinja template test is_active function

ok so what’d we learn? use init_app because app factories be sure to set config defaults calling Flask hooks 0.10+ is a trap how to get to our extension later

ok what now lady

Beyond the basics

http://flask-debugtoolbar.readthedocs.org/

Flask-DebugToolbar

other great extensions Flask-SeaSurf – request processing,

cookies Flask-Admin – SQLAlchemy, blueprints,

static files Flask-Classy – adds class-based views

thanks everybody! @trustrachel github.com/trustrachel trustrachel.com