How can I use OmegaConf custom interpolation with Hydra - fb-hydra

Question

How can I use OmegaConf custom interpolation with Hydra?
Some background:
One can define a custom interpolation for square root:
from omegaconf import OmegaConf
import math
OmegaConf.register_resolver("sqrt", lambda x: math.sqrt(float(x)))
And use it with this config.yaml:
foo: ${sqrt:9}
Loading and printing foo:
cfg = OmegaConf.load('config.yaml')
print(cfg.foo)
Outputs 3.0
When trying this with Hydra:
import hydra
#hydra.main(config_path="config.yaml")
def main(cfg):
print(cfg.foo)
if __name__ == "__main__":
main()
I get the following error:
Unsupported interpolation type sqrt
full_key: foo
reference_type=Optional[Dict[Any, Any]]
object_type=dict
How can I register my resolver when using Hydra?

Answer 1

You can register your custom resolver ahead of time:
config.yaml:
foo: ${sqrt:9}
main.py:
from omegaconf import OmegaConf
import math
import hydra
OmegaConf.register_new_resolver("sqrt", lambda x: math.sqrt(float(x)))
#hydra.main(config_path=".", config_name="config")
def main(cfg):
print(cfg.foo)
if __name__ == "__main__":
main()
This will print 3.0.
This approach will work just as well with the Compose API. The evaluation of the custom resolver is happening when you access the node (lazily).
You just need to register the resolver before you access it.