Hydra in Unit Tests

Use initialize(), initialize_config_module() or initialize_config_dir() in conjunction with compose() to compose configs inside your unit tests.
Be sure to read the Compose API documentation.

The Hydra example application contains an example test.

Testing example with initialize()

from hydra import initialize, compose
# 1. initialize will add config_path the config search path within the context
# 2. The module with your configs should be importable. 
#    it needs to have a __init__.py (can be empty).
# 3. THe config path is relative to the file calling initialize (this file)
def test_with_initialize() -> None:
    with initialize(version_base=None, config_path="../hydra_app/conf"):
        # config is relative to a module
        cfg = compose(config_name="config", overrides=["app.user=test_user"])
        assert cfg == {
            "app": {"user": "test_user", "num1": 10, "num2": 20},
            "db": {"host": "localhost", "port": 3306},
        }

For an idea about how to modify Hydra's search path when using compose in unit tests, see the page on overriding the hydra.searchpath config.