Automatic installation of part dependencies

Buildout parts are requested by the parts option of the buildout section, but a buildout may install additional parts that are dependencies of the named parts. For example, in

develop = .
parts = server

=> app
recipe = zc.zdaemonrecipe
program = ${buildout:bin-directory}/app ${config:location}

recipe = zc.recipe.egg
eggs = myapp

recipe = zc.recipe.deployment:configuration
text = port 8080

the server part depends on the app part to install the server software and on the config part to provide the server configuration.

The config part will be installed before the server part because it’s referenced in a value substitution. The value substitution makes the config part a dependency of the server part.

The server part has the line:

=> app

This line [1], uses a feature that’s new in zc.buildout 2.9. It declares that the app part is a dependency of the server part. The server part doesn’t use any information from the app part, so it has to declare the dependency explicitly. It could have declared both dependencies explicitly:

=> app config

Dependency part selection serves separation of concerns. The buildout parts option reflects the requirements of a buildout as a whole. If a named part depends on another part, that’s the concern of the named part, not of the buildout itself.


The => syntax is a convenience. It’s based on the mathematical symbol for implication. It’s a short hand for:

<part-dependencies> = app

Multiple parts may be listed and spread over multiple lines, as long as continuation lines are indented.