Instantiate resolution and call-site overrides

Hydra 1.4 changes when hydra.utils.instantiate() resolves configuration values and how call-site arguments interact with the input configuration.

Benefits

instantiate() no longer deep-copies and eagerly resolves the full input configuration before it starts instantiating targets. Instead, it traverses the configuration and resolves each value when that value is needed.

This has several benefits:

• Unrelated parts of the configuration tree are not copied or resolved.
• A call-site argument can replace an unresolvable configured value without forcing Hydra to resolve the replaced value first.
• An earlier target can establish runtime state, such as registering a custom resolver, before a later argument is resolved.
• OmegaConf containers passed to a target are still copied, resolved while attached to their original parent, and then detached. This preserves the serialization safety of previous Hydra versions.

Compatibility impact

Call-site arguments are now a separate runtime overlay. They determine the arguments passed to the target, but they do not modify the input configuration or affect how its interpolations resolve.

They are also no longer coerced or validated against the corresponding field in an input Structured Config. They remain runtime inputs, subject to Hydra's normal recursive instantiation and conversion of the supplied value.

For example:

from omegaconf import OmegaConf

from hydra.utils import instantiate

cfg = OmegaConf.create(
    {
        "_target_": "builtins.dict",
        "b": 200,
        "c": "${b}",
    }
)

result = instantiate(cfg, b=99, _target_whitelist_="builtins.dict")
assert result == {"b": 99, "c": 200}

Hydra 1.3 merged b=99 into a copied configuration before resolving ${b}, so c also became 99. Hydra 1.4 leaves cfg unchanged, resolves ${b} against the original configuration, and independently passes the call-site value b=99 to the target.

If another argument should use the call-site value, pass that argument explicitly as well:

result = instantiate(
    cfg,
    b=99,
    c=99,
    _target_whitelist_="builtins.dict",
)