Defaults List

 

You can define a defaults list in your primary Structured Config just like you can in your primary config.yaml file. The example below extends the previous example by adding a defaults list that will load db=mysql by default.

NOTE: You can still place your defaults list in your primary (YAML) config file (Example in next page).

from dataclasses import dataclass

import hydra
from hydra.core.config_store import ConfigStore
from omegaconf import MISSING, OmegaConf

@dataclass
class MySQLConfig:
    ...

@dataclass
class PostGreSQLConfig:
    ...

defaults = [
    # Load the config "mysql" from the config group "db"
    {"db": "mysql"}
]

@dataclass
class Config:
    # this is unfortunately verbose due to @dataclass limitations
    defaults: List[Any] = field(default_factory=lambda: defaults)

    # Hydra will populate this field based on the defaults list
    db: Any = MISSING

cs = ConfigStore.instance()
cs.store(group="db", name="mysql", node=MySQLConfig)
cs.store(group="db", name="postgresql", node=PostGreSQLConfig)
cs.store(name="config", node=Config)


@hydra.main(version_base=None, config_name="config")
def my_app(cfg: Config) -> None:
    print(OmegaConf.to_yaml(cfg))


if __name__ == "__main__":
    my_app()

Running my_app.py loads the mysql config option by default:

$ python my_app.py
db:
  driver: mysql
  ...

You can override the default option via the command line:

$ python my_app.py db=postgresql
db:
  driver: postgresql
  ...

A Note about composition order

The default composition order in Hydra is that values defined in a config are merged into values introduced from configs in the Defaults List - or in other words - overriding them. This behavior can be unintuitive when your primary config is a Structured Config, like in the example above. For example, if the primary config is:

@dataclass
class Config:
    defaults: List[Any] = field(default_factory=lambda:  [
        "debug/activate",
        # If you do not specify _self_, it will be appended to the end of the defaults list by default.
        "_self_"
    ])

    debug: bool = False

And debug/activate.yaml is overriding the debug flag to True, the composition order would be such that debug ends up being False.
To get debug/activate.yaml to override this config, explicitly specify _self_ before debug/activate.yaml:

@dataclass
class Config:
    defaults: List[Any] = field(default_factory=lambda:  [
        "_self_",
        "debug/activate",
    ])

    debug: bool = False

See Composition Order for more information.

Requiring users to specify a default list value

Set db as MISSING to require the user to specify a value on the command line.

Defaults list with a missing db

defaults = [
    {"db": MISSING}
]

Output

$ python my_app.py
You must specify 'db', e.g, db=<OPTION>
Available options:
        mysql
        postgresql