Configuration file

It can get tedious to type all those command line arguments every time. Fix it by creating a configuration file:

Configuration file: config.yaml

db:
  driver: mysql
  user: omry
  pass: secret

Specify the config file by passing a config_path parameter to the @hydra.main() decorator. The location of the config_path is relative to your Python file.

Python file: my_app.py

@hydra.main(config_path='config.yaml')
def my_app(cfg):
    print(OmegaConf.to_yaml(cfg))

config.yaml is loaded automatically when you run your application

$ python my_app.py
db:
  driver: mysql
  pass: secret
  user: omry

You can override values in the loaded config from the command line:

$ python my_app.py db.user=root db.pass=1234
db:
  driver: mysql
  user: root
  pass: 1234

Strict mode

Strict mode is useful for catching mistakes in both the command line overrides and in the code early. Strict mode is on by default when you specify a configuration file for the config_path argument in @hydra.main decorator. It can be turned on or off via the strict argument in your @hydra.main() decorator.

@hydra.main(config_path='config.yaml')
def my_app(cfg):
    driver = cfg.db.driver # Okay
    user = cfg.db.user # Okay
    password = cfg.db.password # Not okay, there is no password field in db!
                               # This will result in a KeyError

Strict mode will also catch command line override mistakes:

$ python my_app.py db.port=3306
Traceback (most recent call last):
...
KeyError: 'Accessing unknown key in a struct : db.port

With strict mode off, accessing unknown keys in the config is permitted and both the above override and the example below would run.

@hydra.main(config_path='config.yaml', strict=False)
def my_app(cfg):
    cfg.db.port = 3306 # Okay