How to define a class type in Hydra, without instantiation?

[FeryET]

I am trying to lazily instantiate a class (from multiple options) which requires a certain object, that only becomes available after an event.

Imagine something liks this:

class Model(self, optim_params):
    super().__init__()
    self.opt = optim_params["cls"](**optim_params["kwargs"])

How can I achieve such thing in Hydra? I want to indicate what class is `optim_params["cls"].

[Jasha10]

If you know ahead of time what the type will be, you could register an OmegaConf custom resolver like this:

from omegaconf import OmegaConf

class Foo:
    ...

def get_Foo():
    return Foo

OmegaConf.register_new_resolver("getFoo", get_Foo)

yaml_data = """
cls: ${getFoo:}
"""

optim_params = OmegaConf.create(yaml_data)
print(optim_params.cls)  # prints <class '__main__.Foo'>
print(type(optim_params.cls))  # prints <class 'type'>

If you do not know the class ahead of time, you can try registering the hydra.utils.get_class method as a custom resolver:

from omegaconf import OmegaConf
from hydra.utils import get_class

class Foo1:
    ...

class Foo2:
    ...

OmegaConf.register_new_resolver("getClass", get_class)

yaml_data = """
cls1: ${getClass:__main__.Foo1}
cls2: ${getClass:${indirection}}
indirection: __main__.Foo2
"""

optim_params = OmegaConf.create(yaml_data)
print(optim_params.cls1)  # prints <class '__main__.Foo1'>
print(optim_params.cls2)  # prints <class '__main__.Foo2'>

In general, OmegaConf's resolvers are a good way to handle "lazy computation" use-cases like this one.