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Version: 1.1

The Defaults List

Introduction#

important

Many of the features described in this page are new. Please report any issues.

The Defaults List is a list in an input config that instructs Hydra how to build the output config. Each input config can have a Defaults List as a top level element. The Defaults List itself is not a part of output config.

Defaults List YAML syntax
defaults:
(- CONFIG|GROUP_DEFAULT)*
CONFIG : (CONFIG_GROUP/)?CONFIG_NAME(@PACKAGE)?
GROUP_DEFAULT : [optional|override]? CONFIG_GROUP(@PACKAGE)?: OPTION
OPTION : CONFIG_NAME|CONFIG_NAMES|null

CONFIG : A config to use when creating the output config. e.g. db/mysql, db/[email protected].

GROUP_DEFAULT : An overridable config. e.g. db: mysql, [email protected]: mysql.

  • override : Overrides the option of a previously defined GROUP_DEFAULT.
  • optional : By default, an OPTION that do not exist causes an error; optional suppresses the error.
  • null : A place-holder for a future override. If it is not overridden the entry is ignored.

CONFIG_NAME: The name of a config, without the file system extension. e.g. mysql and not mysql.yaml.

CONFIG_NAMES : A list of config names. e.g. [mysql, sqlite]

CONFIG_GROUP : A path to a set of configs.
The path is relative to the containing config. It can be made absolute by prefixing it with a /.
The path separator is / regardless of the operating system.

OPTION: The currently selected CONFIG_NAME or CONFIG_NAMES from a CONFIG_GROUP.

PACKAGE : Where to place the content of the config within the output config. It is relative to the Package of the containing config by default. See Packages.

An example#

Config directory structure
โ”œโ”€โ”€ server
โ”‚ โ”œโ”€โ”€ db
โ”‚ โ”‚ โ”œโ”€โ”€ mysql.yaml
โ”‚ โ”‚ โ””โ”€โ”€ sqlite.yaml
โ”‚ โ””โ”€โ”€ apache.yaml
โ””โ”€โ”€ config.yaml

Input configs:

config.yaml
defaults:
- server/apache
debug: false
server/apache.yaml
defaults:
- db: mysql
name: apache
server/db/mysql.yaml
name: mysql
server/db/sqlite.yaml
name: sqlite

Output config:

$ python my_app.py
server:
db:
name: mysql
name: apache
debug: false

Overriding Config Group options#

A Config Group's option can be overridden using a new GROUP_DEFAULT with the override keyword. If a Group Default is overridden more than once, the last one, in depth first order, wins.

Extending the previous example:

config.yaml
defaults:
- server/apache
- override server/db: sqlite
debug: false
$ python my_app.py
server:
db:
name: sqlite
name: apache
debug: false

A Config Group's option can also be overridden via the command line. e.g:

$ python my_app.py server/db=sqlite

Composition order#

The Defaults List is 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.

By default, the content of a config is overriding the content of configs in the defaults list.

config.yaml
defaults:
- db: mysql
db:
host: backup
Result: db.host from config
db:
driver: mysql # db/mysql.yaml
host: backup # config.yaml
port: 3306 # db/mysql.yaml

The _self_ entry determines the relative position of this config in the Defaults List. If it is not specified, it is added automatically as the last item.

config.yaml
defaults:
- _self_
- db: mysql # Overrides this config
db:
host: backup
Result: All values from db/mysql
db:
driver: mysql # db/mysql.yaml
host: localhost # db/mysql.yaml
port: 3306 # db/mysql.yaml

With _self_ at the top of the Defaults List, the host field defined in config.yaml now precedes the host field defined in db/mysql.yaml, and as a result is overridden.

Interpolation in the Defaults List#

Config Group Options can be selected using interpolation.

defaults:
- server: apache
- db: mysql
- combination_specific_config: ${server}_${db} # apache_mysql

Interpolation keys can be config groups with any @package overrides.
For example: ${db/engine}, ${[email protected]}

The selected option for combination_specific_config depends on the final selected options for db and server.
e.g. If db is overridden to sqlite, combination_specific_config will become apache_sqlite.

Restrictions:#

  • Defaults List interpolation is only supported in the primary config.
  • The subtree expanded by an Interpolated Config may not contain overrides.
  • Interpolation Keys in the Defaults List cannot reference values in the Final Config Object (it does not yet).

See Patterns/Specializing Configs for more information.

Debugging the Defaults List#

Hydra's config composition process is as follows:

  • The Defaults Tree is created.
  • The Final Defaults List is created via a DFS walk of the Defaults Tree.
  • The Output Config is composed from the entries in the Final Defaults List.

You can inspect these artifacts via command line flags:

  • --info defaults-tree shows the Defaults Tree.
  • --info defaults Shows the Final Defaults List.
  • --cfg job|hydra|all Shows the Output Config.

Example outputs:

python my_app.py --info defaults-tree
<root>:
hydra/config:
hydra/hydra_logging: default
hydra/job_logging: default
hydra/launcher: basic
hydra/sweeper: basic
hydra/output: default
hydra/help: default
hydra/hydra_help: default
_self_
config:
server/apache:
server/db: mysql
_self_
_self_
python my_app.py --info defaults
Defaults List
*************
| Config path | Package | _self_ | Parent |
-------------------------------------------------------------------------------
| hydra/hydra_logging/default | hydra.hydra_logging | False | hydra/config |
| hydra/job_logging/default | hydra.job_logging | False | hydra/config |
| hydra/launcher/basic | hydra.launcher | False | hydra/config |
| hydra/sweeper/basic | hydra.sweeper | False | hydra/config |
| hydra/output/default | hydra | False | hydra/config |
| hydra/help/default | hydra.help | False | hydra/config |
| hydra/hydra_help/default | hydra.hydra_help | False | hydra/config |
| hydra/config | hydra | True | <root> |
| server/db/mysql | server.db | False | server/apache |
| server/apache | server | True | config |
| config | | True | <root> |
-------------------------------------------------------------------------------
python my_app.py --cfg job
server:
db:
name: mysql
name: apache
debug: false

Related topics#

Last updated on by Jasha10