Talk given at PyTennessee on 2014. Download the original for all the speaker notes.

1. DEVELOPING FLASK EXTENSIONS
Rachel Sanders @ PyTennessee 2014

2. Rachel Sanders
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Engineer at LinkedIn & PyLadiesSF organizer
Our internal stack: Python + Flask
Team lead for a 30K LOC Flask app

3. What we talkin’ about
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
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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

4. The really really quick intro to Flask
Like 3 min tops, promise

5. from flask import Flask
app = Flask(__name__)
@app.route("/")
def hello():
return "Hello San Dimas High!"
if __name__ == "__main__":
app.run()

6. whaaaaaaa?
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

No database??
No forms?
No admin interface?

7. “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

8. All these are extensions


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database  SQLAlchemy + Flask-SQLAlchemy
forms  WTForms + FlaskWTF
admin  Flask-Admin

9. The really quick intro to extending Flask
Like 5 min, promise

10. from flask import Flask
app = Flask(__name__)
@app.route("/")
def hello():
return "Hello San Dimas High!"
if __name__ == "__main__":
app.run()

11. from flask import Flask
app = Flask(__name__)
@app.route("/")
def hello():
return "Hello San Dimas High!"
if __name__ == "__main__":
app.run()

12. extending Flask = changing app

13. You can change the app object by


Hooking into request lifecycle
Adding more resources
 Jinja
filters, tests, global variables
 Routes, blueprints, static files
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
Middleware
Monkeypatching

16. Flask-FeatureFlags
with the power to turn code on or off

17. from flask import Flask
app = Flask(__name__)
@app.route("/")
def hello():
return "Hello San Dimas High!”

18. 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!"

19. 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!"

20. 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!"

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

22. FEATURE FLAG SPEC

feature flags stored in config

Jinja template test called “active_feature”

Module function called “is_active”

24. bit.ly/giantwallofcode

25. class FeatureFlags(object):

26. class FeatureFlags(object):
def __init__(self, app=None):

27. class FeatureFlags(object):
def __init__(self, app=None):
if app is not None:
self.init_app(app)
def init_app(self, app):

28. 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’, {})

29. 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

30. FEATURE FLAGS SPEC

feature flags stored in config

Jinja template test

is_active function

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

32. 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’, {})
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

36. 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

37. FEATURE FLAGS SPEC

feature flags stored in config

Jinja template test

is_active function

38. 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!"

39. 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

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

41. FEATURE FLAGS SPEC

feature flags stored in config

Jinja template test

is_active function

42. ok so what’d we learn?
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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

43. Beyond the basics
ok what now lady

44. Flask-DebugToolbar
http://flask-debugtoolbar.readthedocs.org/

47. other great extensions
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Flask-SeaSurf – request processing, cookies
Flask-Admin – SQLAlchemy, blueprints, static files
Flask-Classy – adds class-based views

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