Config Groups

 

Structured Configs can be used to implement config groups. Special care needs to be taken when specifying a default value for fields populated by a config group. We will look at why below.

Defining a config group for database

from dataclasses import dataclass

import hydra
from hydra.core.config_store import ConfigStore

@dataclass
class MySQLConfig:
    driver: str = "mysql"
    host: str = "localhost"
    port: int = 3306

@dataclass
class PostGreSQLConfig:
    driver: str = "postgresql"
    host: str = "localhost"
    port: int = 5432
    timeout: int = 10

@dataclass
class Config:
    # We will populate db using composition.
    db: Any

# Create config group `db` with options 'mysql' and 'postgreqsl'
cs = ConfigStore.instance()
cs.store(name="config", node=Config)
cs.store(group="db", name="mysql", node=MySQLConfig)
cs.store(group="db", name="postgresql", node=PostGreSQLConfig)

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

if __name__ == "__main__":
    my_app()

caution

The Config class is NOT the Defaults list. We will see the Defaults list in the next page.

You can select the database from the command line:

$ python my_app.py +db=postgresql
db:
  driver: postgresql
  host: localhost
  password: drowssap
  port: 5432
  timeout: 10
  user: postgres_user

The + above is required because there is no default choice for the config group db. The next page will reintroduce the Defaults List, eliminating the need for the +.

Config inheritance

 

Standard Python inheritance can be used to get improved type safety, and to move common fields to the parent class.

Defining a config group for database using inheritance

from omegaconf import MISSING

@dataclass
class DBConfig:
    host: str = "localhost"
    port: int = MISSING
    driver: str = MISSING

@dataclass
class MySQLConfig(DBConfig):
    driver: str = "mysql"
    port: int = 3306

@dataclass
class PostGreSQLConfig(DBConfig):
    driver: str = "postgresql"
    port: int = 5432
    timeout: int = 10

@dataclass
class Config:
    # We can now annotate db as DBConfig which
    # improves both static and dynamic type safety.
    db: DBConfig

Missing fields

Assign MISSING to a field to indicates that it does not have a default value. This is equivalent to the ??? literal we have seen in OmegaConf configs before.

Omitting a default value is equivalent to assigning MISSING to it, although it is sometimes convenient to be able to assign MISSING it to a field.

caution

Do not confuse omegaconf.MISSING with dataclass.MISSING.