Hydra Zen

hydra-zen Fluency

Overview

hydra-zen is not plain Hydra. It's a thin layer on top of Hydra/OmegaConf whose job is to eliminate hand-written YAML and `_target_` strings by generating structured configs from real Python callables. The two things you trade for that:

1. Type safety: configs are dataclasses derived from the callable's signature; missing or misspelled fields fail at config-build time, not at runtime (builds(Foo, momtum=0.9) raises TypeError immediately if Foo has no momtum kwarg). 2. Less boilerplate (DRY): no conf/ YAML tree, no _target_: pkg.mod.Class strings — you pass the class/function itself to builds(...) and hydra-zen mirrors the signature for you. Refactor the target, the config follows automatically.

If you find yourself writing a Hydra-style string where hydra-zen would accept a callable or a builder, you're working around hydra-zen instead of using it.

When to use

Use this skill when you see any of:

• Imports from hydra_zen (builds, make_custom_builds_fn, store, just, MISSING, hydrated_dataclass, kwargs_of, make_config, instantiate, to_yaml, zen, ZenStore, BuildsFn).
• A hydra_defaults=[...] list passed into a store(...) call instead of a defaults: YAML block.
• _target_ strings, ${interpolations}, or MISSING sentinels in a project that uses hydra-zen.
• builds_bases=(Parent,) for config inheritance, or bases=(Cfg,) on make_config.
• LLM-written config that looks like Hydra YAML translated into Python kwargs.
• A "partial of partial" pattern — functools.partial(builds_fn, target, ...) — usually a sign someone was fighting populate_full_signature.

Do not use this skill for:

• Pure Hydra/OmegaConf projects with no hydra_zen import — those follow different rules (YAML is the source of truth).
• Runtime/training logic. This skill is about config construction, not the code that consumes the instantiated objects.

How it works (the typing POV)

Hydra:      YAML files  →  OmegaConf DictConfig  →  hydra.utils.instantiate
hydra-zen:  Python      →  dataclass (via builds) →  hydra_zen.instantiate
                                ↑
                       this is the part you write

builds(Foo, **kwargs) does roughly this at import time:

1. Calls inspect.signature(Foo) to read parameters, defaults, and annotations. 2. Validates each kwarg you passed against that signature — typos → TypeError now, not at runtime. 3. Refines each annotation to something Hydra/OmegaConf accepts (see "Type refinement" below). 4. Generates a @dataclass whose fields mirror the (refined) signature, with _target_ = "pkg.mod.Foo" baked in. 5. Returns that dataclass type. instantiate(Cls(**overrides)) constructs Foo(**overrides).

Static typing: builds' overloads are typed so the returned dataclass advertises BuildsWithSig[type[Foo], P] where P is Foo's ParamSpec. Pyright/mypy can therefore check kwargs to the config class against Foo's signature. With @hydrated_dataclass, the decorator uses `dataclass_transform` so the resulting class behaves like a @dataclass to static checkers — attribute access, frozen-ness, and field types are all visible.

Type refinement — the silent type-widening

Hydra only accepts a narrow subset of annotations: Any, primitives, Enum, nested structured configs, List/Dict of those, Optional, and nested containers (OmegaConf ≥ 2.2). Everything else (Literal, Union[X, Y] of non-primitives, custom protocols, TypedDict, Annotated, etc.) will explode at instantiate time in vanilla Hydra.

hydra-zen automatically broadens unsupported annotations:

• Literal[1, 2] → Any
• List[Literal[1, 2]] → List[Any] (preserves container, drops inner)
• tuple[int, ...] → kept as-is if OmegaConf supports it; else Tuple[Any, ...].

Consequences:

• Your config will instantiate, but OmegaConf will not enforce Literal membership at runtime. If you need that, layer on pydantic/beartype via zen_wrappers= or by decorating the target with @pydantic.dataclasses.dataclass.
• Static checkers still see the original types because hydra-zen attaches them in metadata; you don't lose IDE support — you only lose Hydra-side runtime validation for refined fields.

The builders

pbuilds (as defined in this repo) is just make_custom_builds_fn(populate_full_signature=True, zen_partial=True) — read it as "partial builds with full signature".

The signature-population trap

populate_full_signature=True re-derives every field from the target's __init__ defaults. When combined with builds_bases=(Parent,), any field that was set only on the parent is silently overwritten by the target's own __init__ default.

# Parent sets val_splits=["train-val"]
MMDataCfg = pbuilds(CisGeneRegData, val_splits=["train-val"], ..., builds_bases=(DataCfg,))

# WRONG: populate_full_signature on the child re-derives val_splits from
# CisGeneRegData.__init__, blanking what MMDataCfg set.
MMTuneDataCfg = pbuilds(CisGeneRegData, limit_regions=TUNING_CHROM,
                        builds_bases=(MMDataCfg,))

# RIGHT: restate inherited fields explicitly when you need full-signature population.
MMTuneDataCfg = pbuilds(CisGeneRegData, limit_regions=TUNING_CHROM,
                        val_splits=["train-val"], test_splits=["train-test"],
                        builds_bases=(MMDataCfg,))

# ALSO RIGHT (idiomatic — see "Hacking" section): drop full-signature on the child.
size_builds = make_custom_builds_fn(zen_partial=True)   # no populate_full_signature
MMTuneDataCfg = size_builds(CisGeneRegData, limit_regions=TUNING_CHROM,
                            builds_bases=(MMDataCfg,))

Rule of thumb: `populate_full_signature=True` + `builds_bases` does not compose the way you'd hope. Either restate fields, or use a builder without full-signature population for child variants.

Interpolations vs. builders vs. defaults-list relocation

"${name}" is an OmegaConf interpolation resolved against the composed config tree at instantiate time. It is a str sitting in a field whose annotation is something else, so it punches a hole in the type system that hydra-zen's refinement can't see through. There are two legitimate uses; for everything else there's a better tool.

Legitimate use 1: lazy/derived values

There is no type-safe alternative for these. Interpolations are the right tool.

run_dir   = "${data.dir}/${run.name}"     # compose from other tree paths
git_sha   = "${oc.env:GIT_SHA}"           # resolve env at runtime
timestamp = "${now:%Y-%m-%d}"             # built-in resolvers

Legitimate use 2: wiring a group selection into a nested field (but prefer group@pkg)

You'll see this written with "${...}":

# Works, but not the most idiomatic — loss_fn is annotated nn.Module and
# gets a "${loss}" str at config-build time. Wiring is scattered across nested
# builders instead of collocated.
ModelCfg = pbuilds(CisGeneReg, loss_fn="${loss}", scheduler="${scheduler}", flags="${flags}", ...)

store(name="train", loss=MISSING, scheduler=MISSING, flags=MISSING, model=ModelCfg, ...,
      hydra_defaults=["_self_",
                      {"loss": "pairwise_mse"},
                      {"scheduler": "cosine"},
                      {"flags": "multi_baseline"}])(train)

Prefer the defaults-list package relocation `group@dest.path`: the chosen group config is installed directly at the destination, with no interpolation string in the typed field and all wiring collocated in one place.

# Idiomatic — no "${...}" in any nested builder; ModelCfg has no top-level
# `loss` / `scheduler` / `flags` mirror fields at all.
ModelCfg = pbuilds(CisGeneReg, optimizer=OptimCfg, ...)   # no loss_fn= line

store(name="train", model=ModelCfg, ...,
      hydra_defaults=["_self_",
                      {"loss@model.loss_fn":   "pairwise_mse"},
                      {"scheduler@model.scheduler": "cosine"},
                      {"flags@model.flags":    "multi_baseline"}])(train)
# CLI: python train.py loss@model.loss_fn=gar

Trade-offs:

• The interpolation form requires top-level mirror fields (loss=MISSING, etc.) so that ${loss} has something to resolve. The relocation form doesn't — there is no top-level loss field at all.
• The interpolation form means one group choice can broadcast to multiple destinations (${loss} in many places). The relocation form needs one defaults-list entry per destination. Pick interpolation for fan-out, relocation for clean wiring.
• CLI override grammar differs: loss=pairwise_mse (interpolation form) vs loss@model.loss_fn=pairwise_mse (relocation form). The latter is more verbose but more explicit.

Wrong uses of "${...}" (common LLM mistakes)

Heuristic: if there is no matching top-level field bound to a Hydra group, `"${...}"` is wrong. Interpolations are not generic variables — they reference paths in the composed config. And even when a matching field exists, ask whether group@pkg would be cleaner.

MISSING + defaults list — the explicit-choice pattern

MISSING is only needed when a field exists in the config and must be filled by something other than its dataclass default. With interpolation-style group wiring you need both MISSING and a defaults entry; with `group@pkg` relocation you usually need neither, because the destination field is installed by the defaults list itself.

# Interpolation form: requires the MISSING field + defaults entry pair.
store(name="train",
      loss=MISSING,                                       # required field
      scheduler=MISSING,                                  # required field
      hydra_defaults=["_self_",
                      {"loss": "pairwise_mse"},
                      {"scheduler": "cosine"}])(train)

# Relocation form: no top-level loss/scheduler fields at all; the defaults
# list installs the chosen configs directly at model.loss_fn / model.scheduler.
store(name="train",
      hydra_defaults=["_self_",
                      {"loss@model.loss_fn":      "pairwise_mse"},
                      {"scheduler@model.scheduler": "cosine"}])(train)

Why MISSING matters in the interpolation form: it says "this field must be set". Drop the defaults-list entry and the CLI must pass loss=... every run; drop MISSING and a typo silently leaves the field at its dataclass default.

ZenStore — groups, packages, and the "no Python None" rule

store(group="scheduler")(SomeCfg, name="cosine") registers SomeCfg under group scheduler. CLI: scheduler=cosine.

Useful patterns from the docs:

# Pre-curry a group for repeated entries
db_store = store(group="db")
db_store(Database(name="mysql"))
db_store(Database(name="sqlite"))

# Auto-name from a config attribute (lambda receives the cfg)
auto_name = store(name=lambda cfg: cfg.name)
auto_name(group="server")(Server(name="apache", port=80))

# Push everything into Hydra's global store at app startup
if __name__ == "__main__":
    store.add_to_hydra_store()
    zen(task).hydra_main(config_path=None, config_name="train", version_base="1.2")

You cannot register raw `None` as a group option:

# WRONG — hydra-zen rejects None as a config:
scheduler_store(None, name="none")
# Also wrong on CLI: `scheduler=null`. Hydra parses YAML null as Python None
# *before* the group lookup, raising ValueError.

# RIGHT: wrap None in a builds() of a sentinel function.
def _null_scheduler(*a, **kw): return None
scheduler_store(builds(_null_scheduler), name="none")    # CLI: scheduler=none

Experiment overlays via make_config + _global_ package

The idiomatic way to express "experiment X = base config + a few overrides":

from hydra_zen import make_config

experiment_store = store(group="experiment", package="_global_")
experiment_store(
    make_config(
        hydra_defaults=["_self_", {"override /db": "sqlite"}],
        server=dict(port=8080),
        bases=(Config,),            # inherit from the top-level config
    ),
    name="aplite",
)
# CLI: python app.py +experiment=aplite

package="_global_" means the experiment config replaces the top-level config rather than nesting under it; bases=(Config,) means "inherit all fields then override these"; "override /db" (absolute path) is required because we're not using Hydra's YAML search path.

just() — embed Python objects literally

just(x) produces a config whose instantiate(...) returns x unchanged. Use it for things that aren't directly callable-as-target but need to live inside another builds(...):

# A list of already-built callback configs, embedded into the Trainer config.
TrainerCfg = pbuilds(L.Trainer,
                     callbacks=just([builds(RichProgressBar)]),
                     ...)

# Embedding a functools.partial as an alternative to zen_partial=True:
LogConf = just(functools.partial(logger, format_spec='{0:>8s}'))

Without just, hydra-zen tries to interpret the value as a target and fails.

hydrated_dataclass — when you want static typing

builds(...) returns a class, but pyright/mypy can't see field names without overload tricks. @hydrated_dataclass flips it around — you write a dataclass body and the decorator wires in _target_, _partial_, validation, and type-refinement:

from hydra_zen import hydrated_dataclass
from torch.optim import Adam

@hydrated_dataclass(target=Adam, zen_partial=True, frozen=True)
class BuildsAdam:
    lr: float = 0.01
    momentum: float = 0.9

# pyright now flags both of these:
BuildsAdam(lr="a string")   # bad type
conf = BuildsAdam(); conf.lr = 10.0   # frozen

Misspelled field names raise TypeError at decoration time, just like builds(). Use this when you care about IDE attribute completion or frozen=True static checks.

kwargs_of — "give me a dict matching this signature"

When you need a config that instantiates to a `dict` (not the target itself) whose keys/defaults mirror some function:

from hydra_zen import kwargs_of

DataLoaderKwargs = kwargs_of(DataLoader, zen_exclude=("dataset",), batch_size=32)
# instantiate(DataLoaderKwargs(num_workers=4))
# → {"batch_size": 32, "num_workers": 4, ...}

Useful for passing a **kwargs blob through to a constructor you don't own, or for configs that feed dataclasses.replace-style overlays.

BuildsFn subclass — project-wide builder defaults

If you need stronger conventions than make_custom_builds_fn (e.g. always attach a beartype wrapper, always set hydra_convert="object", always reject unknown kwargs), subclass BuildsFn:

from hydra_zen import BuildsFn

class MyBuilds(BuildsFn):
    _default_dataclass_options_for_builds = {"frozen": True}
    # ... override class methods to inject zen_wrappers, etc.

builds = MyBuilds.builds
pbuilds = MyBuilds.builds  # with zen_partial=True via your own wrapper

Reserve this for genuinely project-wide policy. For most projects, make_custom_builds_fn is sufficient.

Hacking corner cases

The "partial-of-partial" pattern (and why it's usually wrong)

Code that looks like this is a smell:

# This repo's cfg.py:
PartialModelCfg = partial(
    pbuilds,
    CisGeneReg,
    loss_fn="${loss}", optimizer=OptimCfg, scheduler="${scheduler}",
    grad_clip_val=1.0, flags="${flags}", device="cuda", compile_arch=True,
    builds_bases=(ModelCfg,),
)
model_store(PartialModelCfg(d_model=32, nhead=2, n_layers=2), name="tiny")
model_store(PartialModelCfg(d_model=64, nhead=4, n_layers=4), name="small")

What's actually happening: the author wants four small per-size variants. They wrap pbuilds in functools.partial so each call only needs d_model/nhead/n_layers. They also set builds_bases=(ModelCfg,) and restate all the shared kwargs — which is redundant if builds_bases works, but is needed here because pbuilds has populate_full_signature=True and that blanks parent-only fields (see "signature-population trap" above).

Idiomatic refactor — use a second builder without full-signature population for variants:

builds_kw = make_custom_builds_fn()                                 # for required configs
pbuilds   = make_custom_builds_fn(populate_full_signature=True,
                                  zen_partial=True)                  # full sig + partial
variant   = make_custom_builds_fn(zen_partial=True)                  # partial, no full sig

# Base carries every shared kwarg (and full signature → CLI overrideable):
ModelCfg = pbuilds(
    CisGeneReg,
    loss_fn="${loss}", optimizer=OptimCfg, scheduler="${scheduler}",
    grad_clip_val=1.0, flags="${flags}", device="cuda", compile_arch=True,
)

# Variants inherit cleanly; only the differing kwargs are listed.
model_store(variant(CisGeneReg, d_model=32, nhead=2, n_layers=2,  builds_bases=(ModelCfg,)), name="tiny")
model_store(variant(CisGeneReg, d_model=64, nhead=4, n_layers=4,  builds_bases=(ModelCfg,)), name="small")

What changed: variant omits populate_full_signature=True, so builds_bases=(ModelCfg,) cleanly composes — parent fields are preserved, child only adds the size triple. No functools.partial indirection. (Trade-off: variants' configs no longer expose every CisGeneReg.__init__ field on the CLI; the base still does because pbuilds is full-signature, and CLI overrides propagate through inheritance.)

Alternative: make_config overlays

If the variant is purely a CLI overlay rather than a separate target config, make_config(..., bases=(ModelCfg,)) works (see "Experiment overlays" above). Use this for compositional experiments; use builds(... builds_bases=...) for typed variants of a single target.

Embedding a functools.partial directly

When you have an instance of functools.partial and want to register it as a config (e.g. a third-party factory you can't decorate), wrap it in just:

LogConf = just(functools.partial(logger, format_spec='{0:>8s}'))

Equivalent to builds(logger, format_spec='{0:>8s}', zen_partial=True) but lets you build the partial outside hydra-zen.

Adding runtime type validation

zen_wrappers=(validates_with_beartype,) or (validates_with_pydantic,) re-asserts the unrefined types at instantiate time:

from hydra_zen.third_party.beartype import validates_with_beartype

ModelCfg = pbuilds(CisGeneReg, ..., zen_wrappers=validates_with_beartype)

Now Literal[...], custom protocols, and union types are enforced when instantiate runs, despite hydra-zen having broadened them for Hydra.

zen_meta — fields that live in the config but not in the target

For tracking metadata (experiment tags, git SHA) that should appear in the dumped config but not be passed to the target's __init__:

ModelCfg = pbuilds(CisGeneReg, ..., zen_meta={"git_sha": "${oc.env:GIT_SHA}"})

git_sha shows up in to_yaml(ModelCfg) but is stripped before CisGeneReg(**kwargs) is called.

zen_exclude — drop signature parameters

For populate_full_signature=True builds where some kwargs shouldn't be configurable:

builds(DataLoader, populate_full_signature=True, zen_exclude=("dataset", "collate_fn"))

Useful when a parameter is set at runtime (dataset is passed by the trainer), but you still want full sig for the others.

Quick reference

Red flags — STOP and reconsider

• _target_ appears as a string anywhere in a hydra-zen file → use builds(...).
• "${something}" where something is not a top-level field of the same store → either inline the builder, add the missing group/field, or (preferred) replace with a {"something@dest.path": "..."} entry in hydra_defaults.
• "${group_name}" with a matching group_name=MISSING mirror field → works, but consider whether group@dest.path relocation would be cleaner. Use interpolation only when you genuinely need the same choice in multiple destinations.
• A child builds_bases=(Parent,) config "mysteriously" loses a parent's field → it's the populate_full_signature re-derivation; restate the field or use a no-full-sig builder.
• functools.partial(some_builds_fn, target, ...) wrapping a hydra-zen builder → almost always working around (2); refactor with builds_bases + a no-full-sig variant.
• store(group="g")(None, name="off") or CLI g=null failing → wrap None in a sentinel builds().
• New YAML file appearing in a hydra-zen project → almost always wrong; the dataclasses are the source of truth.
• instantiate(cfg) returning a functools.partial you didn't expect → the builder was zen_partial=True; either call it, or rebuild without zen_partial.
• Runtime ValidationError on a Literal/Union field → hydra-zen refined it to Any; if you need enforcement, add zen_wrappers=validates_with_beartype or decorate the target with @pydantic.dataclasses.dataclass.

Common LLM mistakes (and the corrections)

See also

• hydra-zen docs: https://mit-ll-responsible-ai.github.io/hydra-zen/
• DRY rationale: docs/source/explanation/dont_repeat_yourself.rst in the upstream repo
• Type refinement: docs/source/explanation/type_refinement.rst
• Static-typed configs: docs/source/explanation/hydrated_dataclass.rst
• Experiment overlays: docs/source/how_to/configuring_experiments.rst
• Partial configs: docs/source/how_to/partial_config.rst
• Pydantic / beartype runtime checking: docs/source/how_to/pydantic_guide.rst, beartype.rst
• Hydra defaults-list semantics still apply: https://hydra.cc/docs/advanced/defaults_list/
• In this repo, src/gvf_germ_som/train/multigene/cfg.py is the canonical local example; the comments there encode several of the gotchas above.