Alex conrad - Pyramid Tweens (PloneConf 2011)

Tweens Alex Conrad @alexconrad Code Monkeyat SurveyMonkeyPloneConf 2011 Nov 5, 2011

What are Pyramid Tweens?

omg tweens r soooo, like, totally awesome!!!!!!1! lol (totally friend me on twtr) ^^

They are NOT pre-teenager girls(too old for toys, too young for boys)

Pyramid Tweens were named after the word “between”

Pyramid Tweensare middlewares

Pyramid Tweens live betweenPyramid code and your application views

Pyramid Tweens are NOT WSGI middlewares

REQUEST

So why use Tweens?

To apply a common behavior on all requests

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

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

These are not Tweens, but actual@foo decorators, ok?@bardef useless_view(request): return {}

Tweens can be stacked together

Pyramid Tweens vs.WSGI Middlewares

WSGI Tweens middleware Pipelining YES YES Error catching YES YESWSGI compliant YES NO Access to application NO YES (registry, template engine, ...)

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 called at application startup

A tween is called per 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 stack registry: 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 stack registry: the pyramid registry this is a tween

Tween positioning

REQUEST

REQUEST ---------------------- INGRESS ---------------------- ------------------------ MAIN ------------------------

3 ways of positioning tweens - implicit - implicit (hint) - explicit

Implicit positioningfrom pyramid.config import Configurator# ... more importsdef 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()

Implicit positioningfrom pyramid.config import Configurator# ... more importsdef main(global_config, **settings): config = Configurator(...) config.add_tween(useless.useless_tween_factory) config.add_tween(useless.null_tween_factory) # ... more stuff REQUEST return config.make_wsgi_app()

INGRESS useless.null_tween_factory useless.useless_tween_factorypyramid.tweens.excview_tween_factory MAIN

REQUEST INGRESS useless.null_tween_factory useless.useless_tween_factorypyramid.tweens.excview_tween_factory MAIN

Suggested positioning (hint)config.add_tween(useless.useless_tween_factory)config.add_tween(useless.null_tween_factory, over=pyramid.tweens.MAIN, under=useless.useless_tween_factory)

Tween constantspyramid.tweens.INGRESSpyramid.tweens.EXCVIEW pyramid.tweens.MAIN

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

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 requires that you declare ALL tweens.

Listing tweens

$ paster ptweens development.ini#mainPosition 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 HTML into your response

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

Transaction Manager (pyramid_tm)

For every request,it creates a new transaction and 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)

2 pyramid_tm calls: transaction.begin() on request thentransaction.commit() or transaction.rollback()

3the transaction package will then call 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 dont need pyramid_tm. Until you get a stupid Exception outside yourblock (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 !!!lets totally follow each other like, on twitter!!!1! LOOOL :D @alexconrad

Alex conrad - Pyramid Tweens (PloneConf 2011)

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