Douwe van der Meij: AOP in Python API design

See the PyGrunn website for more info about this one-day Python conference in Groningen, The Netherlands.

Aspect oriented programming is more often used in Java than Python. I have used Java before, so I tried using this concept in Python now.

The use case was BioBench, which is one of the sponsors of this conference. It is an application for biogas installations. A farmer enters measurements and gets info from this.

One aspect you need for your API is security. Also statistics/logging, serialization. Probably more. The danger is that those aspects are scattered all over your code base and are tangled, interweaved with other aspects.

Aspect Oriented Programming means: separating your concerns. Put each aspect in a separate module. How can you implement this in pure python? Decorators!

What is an aspect:

• pointcuts, a signature of your code, a regular expression (though I don't use it in my Python code)
• join points: point in your code where the pointcut expression is true, so where the advices should be run
• advices: code to run before or after the join point, or around it.

The security code is an 'around' advice. If a condition is true, we execute the main code, otherwise we skip it and raise an exception.

So for example you end up with:

@secure
@serialize
@statistics
@dispatch
def api_call(...):
    ...

It is good to apply this for aspects that are orthogonal to your main code, like (mostly) security.

Audience: in Zope you can use zcml for aspect oriented programming.