Dectate: advanced configuration for Python code

Dectate is a new Python library. It's geared towards framework authors. It's a meta-framework: a framework you can use to easily construct robust and powerful Python frameworks.

So what's a framework anyway? A framework is a system that you supply with code and then it calls it at the appropriate times. Don't call us, we'll call you!

What does this look like in practice? Let's imagine you're building a web framework, and you want the people that use your framework to provide routes and functions that generate responses for those routes:

def foo_view(request):
    return "Some response!"

This hypothetical web framework then interprets HTTP requests, matches the path of the query with /foo, and then calls the function foo_view to generate the response. Once the response is generated, it sends it back as a HTTP response.

In the abstract, the developer that uses the framework uses it for code configuration: you supply some functions or classes along with some configuration meta data. The framework then uses this code at the appropriate times.

So why would you, the framework author, need a meta framework to implement route? You just create a Python decorator. When it's called you just register the path and the function with some global registry somewhere. Yeah, yeah, "you just", we have heard that before. You could indeed just do that, but perhaps you want more:

  • What if the developer that uses your framework uses route('/foo') in two places? Which one to pick? Does the last one registered win or should this be an error? If the framework should pick the last one, what is the last one? Does this depend on import order?

  • What if there's an error? What if there is some configuration conflict or perhaps your framework decides the developer passed in bad meta data? Ideally you'd like to tell the developer that uses your framework exactly what decorators where have the problem.

  • Perhaps you want to allow reuse: a developer can define a whole bunch of routes and then extend them with some extra routes for particular use cases.

  • Perhaps you want to allow overrides: a developer can define a whole bunch of routes but then override specific ones for particular use cases.

  • Perhaps you want your framework to be extensible with new decorators and new registries. How do you allow this in a way that still allows reuse, overrides and error reporting?

Dectate takes care of all that stuff. It is a documented and well-tested library, and it works for Python 2 and Python 3 code.

Dectate is a spin-off from the Morepath web framework. Morepath is great and you should use it. Morepath has had a sophisticated configuration framework for some years now, but it had grown new features over time, which resulted in a bit of cruft and it also was not well documented. To remedy that and make some other improvements, I've spun it into its own independent library now: Dectate. You can read more about Dectate's history here; Dectate is an expression of many lessons learned over a long time.

It is my hope that Dectate goes beyond Morepath and will be considered by other framework authors. Maybe someone will create a Dectate-based configuration system for other web frameworks such as Django or Flask or Pyramid. Or perhaps someone will use Dectate for some new framework altogether, perhaps one not at all related to the web. Maybe you will! Let me know.


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