extend list from default config in Hydra - fb-hydra

Question

I have a list that is defined in my defaults
configuration file base_list:
list:
- 1
- 2
I know I can override the list values in the config file:
defaults:
- base_list
list:
- 3
- 4
which results
list:
- 3
- 4
However, I look for a way to extend the list, and the desired output is:
list:
- 1
- 2
- 3
- 4
Any idea how to do this?

Answer 1

This is not supported directly.
However, you can achieve the desired behavior using the built-in OmegaConf resolvers oc.dict.*.
Those resolvers allow you to access the keys or values of a config node as a list:
cfg = OmegaConf.create(
{
"workers": {
"node3": "10.0.0.2",
"node7": "10.0.0.9",
},
"nodes": "${oc.dict.keys: workers}",
"ips": "${oc.dict.values: workers}",
}
)
# Keys are copied from the DictConfig:
show(cfg.nodes)
# -> type: ListConfig, value: ['node3', 'node7']
# Values are dynamically fetched through interpolations:
show(cfg.ips)
# -> type: ListConfig, value: ['${workers.node3}', '${workers.node7}']
assert cfg.ips == ["10.0.0.2", "10.0.0.9"]
With this, you can compose a dictionary and have a node that access the values or keys as if they are a list.