Sunday, February 25, 2007

Writing Domain Specific Languages with Python

  • The process of writing DSL is the same in all languages.
    • You write an ideal spec.
    • Then, you try to fit the spec so that it's kind of valid syntax for the language you use.
  • An example of ideal domain language (well, maybe not so ideal, but quite good):

test '/Login':
GetReq(uname = 'a_user',
password = '')
expect_tmpl: 'login.tmpl'
expect_tmpl_has: 'errorMsg'
expect_tmpl_data: { 'uname':'a_user',
'missing_fields' : ['Password']}
expect_session_lacks: 'uid'
  • Now the same but as a valid Python syntax:

GetReq(uname = 'a_user',
password = ''),
expect_tmpl = 'login.tmpl',
expect_tmpl_has = 'errorMsg',
expect_tmpl_data = { 'uname':'a_user',
'missing_fields' : ['Password']},
expect_session_lacks = 'uid')

  • Useful tricks inclue:
    • keyword args (expect_tmpl_data=...)
    • unbounded arg lists (*args)
    • operator overloading __add__(self, other_form)
    • reflection via hasattr, getattr, vars and friends
  • Thanks to Dan Milstein for his talk at PyCon about Little Languages in Python.

IronPython and Windows Forms talk

The presentation in html version

TabbedImages application

  • We got some good feedback after the presentation.
  • To my surprise there were many people already playing with IronPython.
  • What's even more surprising, there were some people that have already used Windows Forms.
  • Watch the tabbedimages application, I'm going to extend it with some interesting features.
  • If you browse the sources for it, you'll find some simple tests in the
  • They exist because I'm afraid I'm not able to not do Test Driven Development.

Saturday, February 24, 2007

PyCon 2007

The PyCon conference is really great.
So far I was at the following talks:
  • Internationalization
  • Parsing revisited
  • Developing desktop applications with Dabo
    • I didn't know that creating applications with wxPython is such a pain.
    • The Dabo core is simply a layer above the wxPython.
    • The application they used on the presentation was an image viewer.
    • The API was really nice and in many cases reminded me a simplified version of WinForms API (where you uses strings instead of overused in .Net enumerations).
    • wxPython (so Dabo as well) works on all important platforms (Windows, Linux, MacOS)
  • WSGI - an introduction
    • Because there are many different Python web frameworks, WSGI goal is to be able to integrate different layers of different applications.
    • I'm still not sure how it could be used in the Ruby world and if there is any need for that.
  • Writing parsers and compilers with PLY
    • Recently, I'm working quite a lot with parsers and PLY is the main tool we use.
    • It's very good, very efficient, very elegant
    • You declare your grammar rules using docstrings
  • Creating the WhatWhat project with TurboGears
    • WhatWhat is a very simple project tracking tool that was implemented in 4 days using TurboGears.
    • It seems that frameworks like TurboGears are becoming quite easy to learn and use.
    • Not sure if as easy as Ruby on Rails, though.
    • I'm still not convinced what could be the reason for me not to use Rails in web applications :)
  • pyweek: making games in 7 days
  • Creating games with Pygame on the GP2X
  • SQLAlchemy
    • Looks like a very powerful ORM tool for Python.
    • Reminds me of Hibernate.
    • There is a tool called Elixir which is a layer on top of SQLAlchemy and is more similar to the Ruby ActiveRecord.
    • Migrations are only available as extensions.
    • However, they are different than Ruby migrations. I didn't quite understand what is really the difference. I was told that Ruby migrations are dangerous because they do too many things for you (!?).
    • It seems to me the above is one of the fundamental differences between Python and Ruby programmers.
  • IronPython: Present and future
    • That was definitely the best talk so far, IMO.
    • Jim is doing a great work in Microsoft.
    • My name even appeared at his slide :-)
    • He wasn't able to read my name, though ;-)
    • What's so difficult with my name? - Andrzej Krzywda
    • There were screenshots of Michael's tutorial on IronPython and Windows Forms.
    • Seo was mentioned, of course. He is the author of FePY - the Comunnity Edition of IronPython.
    • The reason of this special edition is that MS can't easily apply patches that are sent by non-MS people.
    • IronPython is supported by the MS Robotics project.
    • Jim presented a small IP script where he used some MS Gaming API to create a simple 3d game.
    • There is a growing support for IronPython in ASP.NET
  • Embedding Little Languages in Python
    • Little Languages is what the rest of the world calls Domain Specific Languages.
    • The talk was about how to use Python to create such a DSL.
    • The idea is the same as always with DSL:
      • Firstly, you think of an ideal specification.
      • You try to use the language features to fit to the spec.
    • Python features that are helpful:
      • Keyword args
      • * args
      • getattr, hasattr
    • The speaker used _ convention in his code examples.
    • I also prefer it, but it doesn't seem to be so popular in the Python community (it is in Ruby community).