Alex conrad - Pyramid Tweens (PloneConf 2011)

Alex Conrad@alexconrad

Code Monkeyat SurveyMonkey

PloneConf 2011Nov 5, 2011

Tweens

What are Pyramid Tweens?

omg tweens r soooo, like, totally awesome!!!!!!1! lol

(totally friend me on twtr)

^^

They are NOTpre-teenager girls

(too old for toys, too young for boys)

Pyramid Tweens were namedafter the word “between”

Pyramid Tweensare middlewares

Pyramid Tweens live betweenPyramid code and your application views

Pyramid Tweens are NOTWSGI middlewares

REQUEST

So why use Tweens?

To apply a common behavioron all requests

Think of them as decoratorsapplied to each of your Pyramid views.

@foo@bardef useless_view(request): return {}

@foo@bardef useless_view(request): return {}

These are notTweens, but actualdecorators, ok?

Tweens can be stacked together

Pyramid Tweensvs.

WSGI Middlewares

WSGI middleware Tweens

Pipelining YES YES

Error catching YES YES

WSGI compliant YES NO

Access to application

(registry, template engine, ...)

NO YES

How does it work?

Create a tween factory: a function

A tween factory takes 2 arguments:

- handler- registry

A tween factory returns a function:a tween

(just like a decorator does)

The tween function takes 1 argument:- request

(just like a view would)

The tween function returns a response object

(just like a view would)

A tween factory is calledat application startup

A tween is calledper request

The tween factory decidesif the tween should be part of the tween stack

def useless_tween_factory(handler, registry): if registry.settings['useless'] == '1': def useless_tween(request): response = handler(request) return response return useless_tween return handler

handler: the next tween in the stackregistry: the pyramid registry

def useless_tween_factory(handler, registry): if registry.settings['useless'] == '1': def useless_tween(request): response = handler(request) return response return useless_tween return handler

handler: the next tween in the stackregistry: the pyramid registry

this is a tween

Tween positioning

REQUEST

---------------------- INGRESS ----------------------

------------------------ MAIN ------------------------

REQUEST

3 ways of positioning tweens

- implicit- implicit (hint)

- explicit

Implicit positioning

from pyramid.config import Configurator

# ... more imports

def main(global_config, **settings):

config = Configurator(...)

config.add_tween('useless.useless_tween_factory')

config.add_tween('useless.null_tween_factory')

# ... more stuff

return config.make_wsgi_app()

REQUEST

Implicit positioning

from pyramid.config import Configurator

# ... more imports

def main(global_config, **settings):

config = Configurator(...)


config.add_tween('useless.null_tween_factory')

# ... more stuff

return config.make_wsgi_app()

INGRESSuseless.null_tween_factory

useless.useless_tween_factorypyramid.tweens.excview_tween_factory

MAIN

REQUEST

INGRESSuseless.null_tween_factory

useless.useless_tween_factorypyramid.tweens.excview_tween_factory

MAIN



- explicit

Suggested positioning (hint)


config.add_tween('useless.null_tween_factory',

over=pyramid.tweens.MAIN,

under='useless.useless_tween_factory')

Tween constants

pyramid.tweens.INGRESSpyramid.tweens.EXCVIEWpyramid.tweens.MAIN

You cannot position a tweenbefore INGRESS (over)nor after MAIN (under)



- explicit

Explicit positioning

# development.ini

[app:main]

...

pyramid.tweens = useless.useless_tween_factory

useless.null_tween_factory

pyramid.tweens.excview_tween_factory

WARNING:

Explicit tween ordering(via the .ini file)

will ignore any calls toconfig.add_tween( … )

WARNING:

[app:main]...pyramid.includes = pyramid_debugtoolbarpyramid.tweens = useless.useless_tween_factory useless.null_tween_factory pyramid.tweens.excview_tween_factory

Explicitly tween positioning requiresthat you declare ALL tweens.

Listing tweens

$ paster ptweens development.ini#main

Position Name INGRESS0 useless.useless_tween_factory1 useless.null_tween_factory2 pyramid.tweens.excview_tween_factory MAIN

Tweens you might be using:

- pyramid_debugtoolbar- pyramid_tm

pyramid_debugtoolbar

Debug Toolbar renders and injects HTMLinto your response

Although it can decide whether or not it should render HTML depending on your host address

Transaction Manager(pyramid_tm)

For every request,it creates a new transactionand closes it on response.

Transaction Manageris NOT about database transactions

pyramid_tm relies on the transaction package

1

Register your commit / rollback action withthe transaction package (join() the transaction)

2pyramid_tm calls:

transaction.begin() on requestthen

transaction.commit() or transaction.rollback()

3

the transaction package will thencall subscribed callbacks

Why is this useful?

One single point of commit.

Avoid sending an email“Your account has been created! \o/”

when your DB write actually failed

Things you could do:

- commit your DB transaction- invalidate cache- send an email

...

I already have try/except blocks in my Pyramid views, I don't need pyramid_tm.

Until you get a stupid Exception outside your block (e.g., in the renderer) for whatever reason.

Also know as: a bug.

Pyramid registers SQLAlchemy automatically

extension=ZopeTransactionExtension()

Did you know?

pyramid_tm is responsiblefor the existence of Pyramid tweens.

It was the first use case for tweens.

Now you are tween experts!

Use wisely.

Thank you !!!

let's totally follow each other like,on twitter!!!1! LOOOL :D

@alexconrad

Alex conrad - Pyramid Tweens (PloneConf 2011)

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