Wednesday, May 12, 2010

list as default value in class __init__() - watchout for surprise

Note to self: don't use list as a default value in class methods if its going to be modified later, as it might cause some trouble.

Just realized something:

In [1]: class MyClass(object):
...: def __init__(self,data=[]):
...: = data
...: def addData(self,value):

In [2]: obj = MyClass()

In [3]: obj.addData('hello')

In [4]:
Out[4]: ['hello']

In [5]: obj2 = MyClass()

In [6]:
Out[6]: ['hello']

In [7]: obj3 = MyClass([])

In [8]:
Out[8]: []

In [12]: def myfunc(val,data=[]):
....: data.append(val)
....: return data

In [13]: myfunc(1)
Out[13]: [1]

In [14]: myfunc(2)
Out[14]: [1, 2]

In [15]: myfunc(3)
Out[15]: [1, 2, 3]

At first i thought its a bug/weirdness, but after banging my head a bit to wall and a long stare at the ceiling, it made sense.

During class definition, the list object already instantiated. Therefore, the default value of data variable is a reference to a list object, rather than a new list on each instantiation.

Not quite sure how to explain this in words ..

In [1]: class MyClass(object):
...: def __init__(self,data=[]): # data->List object at 0x0001
...: = data #>List object at 0x0001
...: def addData(self,value):
...: #>List object at 0x0001

In [2]: obj = MyClass()

In [3]: obj.addData('hello')

In [4]: #>List object at 0x0001
Out[4]: ['hello']

In [5]: obj2 = MyClass()

In [6]: #>List object at 0x0001
Out[6]: ['hello']

In [7]: obj3 = MyClass([]) # []->List object at 0x0002

In [8]: #>List object at 0x0002
Out[8]: []

I think you got the idea.

Saturday, May 08, 2010

Using deliverance as WSGI pipeline to elegantly theme a repoze.bfg project

I was looking around for template inheritance solution for repoze.bfg, as it seems like by default, BFG does not have context view traversal support to acquire template macros.

In Plone/Grok template inheritance can be done by registering main_template as a view, and using a template that fill the slots in main_template such as this:
<html xmlns:tal=""
<div metal:fill-slot="main">
Some content

However, by default, repoze.bfg does not support getting view macro from a context. So a PageTemplate object need to by passed by a view in order to use the macro from the template. Eg:
from repoze.bfg.chameleon_zpt import get_template

def my_view(context,request):
main_template = get_template('templates/')
return dict(main_template=main_template)

So the view would be using:
<html xmlns:tal=""

Simple enough, but something came up my mind. "How about I theme this using Deliverance?"

What is Deliverance?

In simple terms, its a theming proxy which also available as a WSGI middleware. It allow developers to theme different systems without the need to know the internal of the systems. More information here.

Setting up Deliverance in repoze.bfg buildout

So lets get to the fun stuff.

I'm assuming that repoze.bfg is installed in a buildout, and a project called 'helloworld' is installed in the buildout, similar to the one I've shown in my previous post

First, add the egg:
eggs =

and run ./bin/buildout -vvv

Now, configure the Deliverance filter.

Open src/helloworld/helloworld.ini , rename [app:main] to [app:helloworld] and add these:
use = egg:Deliverance#main
theme_uri = /static/layout.html
rule_uri = /static/rules.xml

pipeline =

Afterward, create a layout.html and a rules.xml in helloworld/templates/static with these contents:

<title>The Theme</title>
<h1>A theme</h1>
<div id="content">

<replace content="children:body" theme="children:#content" if-content="not:#content"/>
<replace content="children:#content" theme="children:#content"/>

Documentation of the rules markup are here:

Now you're ready for profit :D

Start the server using ./bin/paster serve src/helloworld/helloworld.ini

Happy Hacking!

repoze.bfg - Introduction and bootstrapping it on buildout

2 months without a post here .. T_T ... must .. restart .. blogging ...

Anyway, I've been poking around with repoze.bfg since last week and growing to love it by the days. Been thinking to post a blog on getting started on it but was a bit lazy .. until I saw lowkster's post about bfg at a few days ago.

Full documentation at

A little review

I love Zope Component Architecture because the modularity/reusability it offers, but Bluebream, Grok and Zope2 feels a bit big for simple web apps or for introducing ZCA to new people. Then I saw BFG.

BFG simplifies many concepts which I'm familiar with in Zope2/Zope3, and it does it without overwhelming developers with other parts of the framework. Not endorsing any type of storage backend, and giving an option between URL routing and traversal or a mix of both is a plus.

The initial base code is simple and straightforward, and there are no need to subclass any parent class when you want to create your initial project.

The starter project is as simple as:
class MyModel(object):

root = MyModel()

def get_root(request):
return root
def my_view(request):
return {'project':'helloworld'}

A registration of the view in configure.zcml:

And its template file which uses TAL compatible markup.

As the initial requirement is simple, and theres close to no enforcement on how developers supposed to write something on it, it made it easy for new people to quickly learn it and start being productive. BFG too, being something that uses many Zope concepts, can utilize many existing Zope/Z3C components out there, and if a developer know how to utilize ZCA well, what developed on a BFG project may also be componentized and reused in other frameworks.

What to know more on whats cool with BFG? .. read their Sales Pitch ;)

Installation on Buildout

Depending on your distribution, BFG might be available in your distro repository (Fedora have it). However, being a Zope/Plone developer as my main job, whenever it comes to installing python applications from pypi, I tend to prefer to use buildout to create a self-contained environment. So I'll use buildout here too.

First, if you don't have zc.buildout yet, install it using:
$ easy_install zc.buildout

Now lets create the contained environment
$ mkdir -p ~/repozebuildout/src
$ cd ~/repozebuildout

Then create a buildout.cfg file in the directory with this config:
parts =
develop =
versions = versions

recipe = zc.recipe.egg
eggs =
interpreter = python
entry-points = paster=paste.script.command:run

repoze.bfg = 1.2.1

What the buildout config will do is, it will create a buildout with repoze.bfg framework and its dependencies installed, with an interpreter script configured for the buildout environment, and a paster script.

Now initialize the buildout
$ buildout init
$ ./bin/buildout -vv

After the buildout initialization is done, you may start creating the project skeleton.

Creating your first project

There are several templates to choose from: bfg_starter, bfg_routesalchemy, bfg_alchemy, and bfg_zodb.

bfg_starter template simply give a very basic bfg skeleton to get started. Those who are familiar with Pylons/Django might want to look at bfg_routesalchemy and those who are familiar with Zope might want to look at bfg_alchemy and bfg_zodb.

For this example, i'll just create a simple project using bfg_starter template.

cd src/
../bin/paster create -t bfg_starter helloworld
cd ..

then , edit buildout.cfg and add helloworld into the eggs section:

eggs =

afterward, rerun buildout
./bin/buildout -vvv

Once that is done, you may start the server using:
./bin/paster serve src/helloworld/helloworld.ini

Now you can start developing on BFG!. Read the documentation to get you started in developing on BFG.

Happy hacking :D
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