Python-based configuration in a Grok near you

  • By Martijn Faassen
  •  • 
  • 2010-01-02
  •  • 
  • Tags: 
  • grok
  • framework

BFG 1.2 offers imperative configuration: doing configuration not in ZCML but in Python. The declarative configuration system is built on top of this. This is an interesting approach that has some merit.

It's interesting to compare this to Grok's configuration system, which also focuses on Python, not ZCML. Grok however offers a declarative configuration system in Python, not an imperative one.

It's important to note that from the start Grok's declarative configuration system allowed the user to be entirely explicit about how configuration happens. I don't know whether Chris was referring to Grok when he mentioned that in BFG no false choice between convention over configuration and ZCML is offered, but in case anyone had the wrong impression: Grok doesn't offer such a choice. You can be as explicit as you like. Relying on convention-based configuration patterns is an option when using Python-based configuration, but not an obligation.

Grok's declarative configuration system can be used in tests. If you have a component that you need to register, you can use grok.testing.grok_component to automatically configure it according to the standard Grok configuration rules.

Here is an example, a component:

import grok
class MyAdapter(grok.Adapter):

This adapter when configured, be registered as allowing instances of class AdaptedFrom to objects providing interface IAdaptedFrom.

If we want to configure this adapter in our tests, we can use grok_component:

>>> from grok.testing import grok_component
>>> grok_component('MyAdapter', MyAdapter)

The grok_component function can also be imported from the reusable grokcore.component library, through grokcore.component.testing.

Interesting about this design is that Grok allows the use of the same configuration system for test-configuration as well as real configuration. There is no need to learn a different configuration approach when running tests.

Grok's focus on configuration as an area to challenge existing Zope assumptions is long-standing. Grok strikes a different balance than BFG as Grok aims to remain compatible with existing Zope-based systems.

Nothing's perfect. So, I'll close with some areas where we should improve Grok's configuration system:

  • while it is very convenient to be able to register a component with the same configuration as it would have in a real application (a common case), we might need a way to support registering an existing component differently. Subclassing probably usually does provide this.
  • we need to better promote grok_component as a well-understood tool by writing better documentation and more tests. Often I myself fall back on using the zope.component registration APIs in tests directly myself, and I need to understand why. Perhaps this is simply something to get used to, or perhaps the reasons are more fundamental and there is a BFG-like imperative configuration system at some level.
  • we need to fix the API so we can avoid the first argument to grok_component, which is, if I recall it correctly, mostly useless.