[Python-Dev] proto-pep: plugin proposal (for unittest)

[Michael Foord]

On 30/07/2010 21:56, P.J. Eby wrote:
> At 03:34 PM 7/30/2010 +0100, Michael Foord wrote:
>> Automatic discoverability, a-la setuptools entry points, is not 
>> without its problems though. Tarek outlines some of these in a more 
>> recent blog post:
>
> FWIW, it's not discovery that's the problem, but configuring *which* 
> plugins you wish to have active. Entry points support access by name, 
> and it's up to the application using them to decide *which* ones to load.
>
> The underlying idea is that entry points expose a hook; it's up to the 
> app to decide which ones it should actually import and use. An 
> application also can list the available plugins and ask the user, etc. 
> (For example, setuptools only loads "setup() argument" entry points 
> for specified arguments, and command entry points only for the 
> commands a user explicitly invokes.)
>
> IOW, entry points provide access to plugins, not policy or 
> configuration for *which* plugins you wish to use. This was an 
> intentional decision since applications vary widely in what sort of 
> configuration mechanism they use. In the simplest cases (e.g. 
> single-app environments like Chandler), simply making the plugin 
> available on sys.path (e.g. via a special plugins directory) is 
> configuration enough. In more complex use cases, an app might have to 
> import plugins in order to get more information about them.
>

Right (and thanks), and in the unittest plugin system the user decides 
which plugins should be active by listing them explicitly in the 
configuration file.

Discovery could be useful for a tool that tells the user which plugins 
are available and modify the config file *for them* to switch plugins on 
and off. Useful metadata would then be which config options a plugin 
supports (and their defaults) so they can be added to the config file 
too when a plugin is activated.

Michael

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