Packages should load configuration they depend upon

Summary and current situation

When we "exploded" Zope into individual eggs, we introduced explicit package dependencies in setup.py. This allows us to install a package indepedently and still ensure that its dependencies are installed as well.

Packages not only depend on other Python packages because they want to import their components, however. They often also depend on those package's configuration. Therefore, in order to be able to run the functional tests of each package, we introduced functional test layers for each package which load the necessary configuration.

The following proposal outlines why not the functional test layer should load the configuration that the package depends on, but the package itself.

Problem

Let's say you want to use any package that requires more configuration than it currently loads. A good example is zope.app.apidoc. You can't simply say:

<include package="zope.app.apidoc" />

Instead, you'll have to include the configuration of all its dependencies first:

<include package="zope.app.zcmlfiles" file="meta.zcml" />
<include package="zope.app.preference" file="meta.zcml" />
<include package="zope.app.onlinehelp" file="meta.zcml" />
<include package="zope.app.apidoc" file="meta.zcml" />

<include package="zope.app.zcmlfiles" />
<include package="zope.app.tree" />
<include package="zope.app.onlinehelp" />
<include package="zope.app.renderer" />
<include package="zope.app.preference" />
<include package="zope.app.apidoc" />

Proposed solution

For each package, move the include statements that are intrinsic to the package's dependencies from ftesting.zcml to configure.zcml. For instance, zope.app.apidoc/configure.zcml would start off like this:

<configure>
  <include package="zope.app.zcmlfiles" file="meta.zcml" />
  <include package="zope.app.preference" file="meta.zcml" />
  <include package="zope.app.onlinehelp" file="meta.zcml" />
  <include file="meta.zcml" />

  <include package="zope.app.zcmlfiles" />
  <include package="zope.app.tree" />
  <include package="zope.app.onlinehelp" />
  <include package="zope.app.renderer" />
  <include package="zope.app.preference" />

Then, if you wanted to enable APIDoc? in your application, you could simply do:

<include package="zope.app.apidoc" />

Risks

You might want to load a package's configuration without loading the configuration that the package depends on, for instance because you want to supply a different set of components/directives. This can still be achieved with the proposed model by using the exclusion mechanism provided by zc.configuration.

Not a risk: Since most packages share common requirements regarding the configuration they depend on (e.g. the meta directives that register the browser directives), some ZCML files will likely be included multiple times. This is not a problem though because ZCML will only execute the file the first time and then ignore subsequent includes.