The configuration syntax in the new version of zc.buildout is extended with two operators += and -= to allow adjusting options in an extended profile. This is very helpful. You don't need anymore to overwrite an option in the base configuration when all you want is to remove/add an entry.

here is an example:

# base.cfg file
[buildout]

parts =
part1
part2

eggs =
eggs1
eggs2
eggs3

To add new part and remove egg2 in an extended configuration, we do:

# newconfig.cfg file

[buildout]
extends = base.cfg

parts +=
part3

eggs -=
eggs2

Running buildout, the result will be as if you have run it with this configuration file:

[buildout]
parts =
part1
part2
part3

eggs =
egg1
eggs3


That's it.
If you want to add new egg to all your profiles all you have to do is to add it to the base.cfg.