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Selecting default configs

 Example

After office politics, you decide that you want to use MySQL by default. You no longer want to type +db=mysql every time you run your application.

You can add a Default List to your config file. A Defaults List is a list telling Hydra how to compose the final config object. By convention, it is the first item in the config.

Config group defaults#

config.yaml
defaults:  - db: mysql

Remember to specify the config_name:

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

When you run the updated application, MySQL is loaded by default.

$ python my_app.pydb:  driver: mysql  pass: secret  user: omry

You can have multiple items in the defaults list, e.g.

defaults: - db: mysql - db/mysql/engine: innodb

The defaults are ordered:

  • If multiple configs define the same value, the last one wins.
  • If multiple configs contribute to the same dictionary, the result is the combined dictionary.

Overriding a config group default#

You can still load PostgreSQL, and override individual values.

$ python my_app.py db=postgresql db.timeout=20db:  driver: postgresql  pass: drowssap  timeout: 20  user: postgres_user

You can remove a default entry from the defaults list by prefixing it with ~:

$ python my_app.py ~db{}

Non-config group defaults#

Sometimes a config file does not belong in any config group. You can still load it by default. Here is an example for some_file.yaml.

defaults:  - some_file

Config files that are not part of a config group will always be loaded. They cannot be overridden.
Prefer using a config group.

info

For more information about the Defaults List see Reference Manual/The Defaults List.