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Version: 1.0

Grouping config files

Β Example (Click Here)

Suppose you want to benchmark your application on each of PostgreSQL and MySQL. To do this, use config groups.

A Config Group is a named group with a set of valid options.

  • The config options are mutually exclusive. Only one can be selected.
  • Selecting a non-existent config option generates an error message with the valid options.

To create a config group, create a directory. e.g. db to hold a file for each database configuration option. Since we are expecting to have multiple config groups, we will proactively move all the configuration files into a conf directory.

Directory layout
β”œβ”€β”€ conf
β”‚Β Β  └── db
β”‚Β Β  β”œβ”€β”€ mysql.yaml
β”‚Β Β  └── postgresql.yaml
# @package _group_
driver: mysql
user: omry
password: secret

The config group determines the package of the config content inside the final config object.

Interpretation of db/mysql.yaml
driver: mysql
user: omry
password: secret

In Hydra 1.1 _group_ will become the default package.
For now, add # @package _group_ at the top of your config group files.
Learn more about packages directive here.

Using config groups​

Since we moved all the configs into the conf directory, we need to tell Hydra where to find them using the config_path parameter. config_path is a directory relative to
def my_app(cfg: DictConfig) -> None:

Running without requesting a configuration will print an empty config.

$ python

You can append an item a config group to the Defaults List.
The Defaults List is described on the next page.

$ python +db=postgresql
driver: postgresql
pass: drowssap
timeout: 10
user: postgres_user

Like before, you can still override individual values in the resulting config:

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

More advanced usages of config groups​

Config groups can be nested. For example the config group db/mysql/storage_engine can contain innodb.yaml and myisam.yaml. When selecting an option from a nested config group, use /:

$ python +db=mysql +db/mysql/storage_engine=innodb
driver: mysql
user: omry
password: secret
innodb_data_file_path: /var/lib/mysql/ibdata1
max_file_size: 1G

This simple example also demonstrated a very powerful feature of Hydra: You can compose your configuration object from multiple configuration groups.