Configuration Conventions

Core Rule

Defaults must not be scattered at call sites. A default for a config value must live in exactly one place; readers should never have to grep through algo code to discover what value was actually used. Where that one place is depends on the schema type — see "Where Defaults Live" below — but it is never a cfg.get("k", default), a function-parameter default, or a magic constant inside the consumer.

Class Choice: BaseModel vs. dataclass vs. TypedDict

We are gradually migrating the config schema from typing.TypedDict (v1) to pydantic.BaseModel (v2). Both styles coexist in the codebase today, and the migration is tracked by tests/unit/test_config_v2.py against the reference configs in tests/unit/reference_configs/ (these will be removed when the migration is complete).

Use the right tool for the job. v2 (the new convention):

• pydantic.BaseModel — v2, user-facing config (the new default). Any class the user touches via YAML — currently the top-level MasterConfig of each algorithm (grpo, dpo, sft, rm, distillation, eval) and a few shared schemas like ClippedPGLossConfig — is a BaseModel declared with extra="allow" so unknown keys don't break older configs:

  from pydantic import BaseModel
  
  class MasterConfig(BaseModel, extra="allow"):
      policy: PolicyConfig
      grpo: GRPOConfig
      ...

  Prefer BaseModel for new user-facing config classes, and when converting an existing TypedDict as part of the v1 → v2 migration.

• @dataclass — v2, internal classes (not loaded from YAML). For purely in-process data containers — worker metadata, datum specs, internal state passed between Python components — use @dataclass (e.g. nemo_rl/distributed/worker_groups.py, nemo_rl/data/interfaces.py, nemo_rl/data_plane/interfaces.py). Do not use BaseModel or TypedDict for these — they're not config and shouldn't pretend to be.

v1 (legacy, being migrated away):

• typing.TypedDict — v1, legacy / not-yet-migrated user-facing config. Some nested sub-configs are still TypedDict. Continue to maintain them with the same defaults rules below until they are migrated to BaseModel. Use typing.NotRequired to mark optional attributes. Do not add new TypedDict-based config classes.

When in doubt: is this class populated from a user-edited YAML? If yes → BaseModel (or legacy TypedDict). If no → @dataclass.

New code follows v2 directly

Any newly added class must follow the new convention from the start:

• New user-facing config → pydantic.BaseModel (with extra="allow" when user configs may carry extra/obsolete keys). Do not add new TypedDict-based config classes.
• New internal class → @dataclass.

TypedDict edits should only happen on existing, not-yet-migrated classes — either to extend them or as part of converting them to BaseModel.

Access Config Directly

For required attributes, read the value and assume it is present — don't introduce a fallback at the call site.

• v2 (BaseModel): attribute access — master_config.policy.precision. The BaseModel class itself supplies any default for the field; the call site reads it as-is.
• v1 (TypedDict, omegaconf-loaded dict): key access — policy_cfg["precision"]. The exemplar YAML supplies the default; the call site reads it as-is.

In both cases, missing required values should fail loudly at load/access time rather than being silently papered over.

Express Optionality

• TypedDict: use typing.NotRequired[...] to mark optional attributes.
• BaseModel: declare the field as Optional[...] = None (or T | None = None).

Optional attributes may be absent/None; code may check for their presence. Never substitute a non-None default at the access site (see "Accessing NotRequired Fields" below).

Where Defaults Live

The location of defaults depends on the schema type:

• v2 — pydantic.BaseModel (user config): the default lives on the BaseModel field as a Python value. The BaseModel class is the centralized source of truth for user-facing defaults — exemplar YAML serves as documentation / override examples, not as the canonical default store. Example (from ClippedPGLossConfig):

  class ClippedPGLossConfig(BaseModel, extra="allow"):
      disable_ppo_ratio: bool = False
      ratio_clip_min: float = 0.2
      ratio_clip_c: Optional[float] = None  # None to disable
      reference_policy_kl_penalty: float = 0.01

• v1 — typing.TypedDict (legacy user config, pre-migration): the default lives only in the exemplar YAML under examples/configs/*.yaml. There is no class-level default — Python code must read the value from the loaded dict without supplying a fallback.
• @dataclass (internal class, not loaded from YAML): usually no defaults at all. Fields are populated by the producing code path; a stray = None / field(default=...) is a smell unless there's a clear reason (e.g., forward-compat for an optional field added mid-migration).

In all three cases:

• Exemplar configs under examples/configs/*.yaml include documented defaults. For v1 TypedDict configs they are the source of truth; for v2 BaseModel configs they serve as documentation and reasonable starting points, with the BaseModel class itself being authoritative.
• Recipe YAMLs under examples/configs/recipes/**/*.yaml are runnable snapshots and may omit documentation.
• Defaults at call sites (cfg.get("k", default), function-parameter defaults, magic constants) are never allowed — see "Forbidden Patterns" below.

Documenting New Config Keys

When adding a new config key to a BaseModel or TypedDict subclass, document:

• The key's purpose
• Valid values/types
• Recommended default (if applicable)

Reflect the default in the exemplar YAMLs under examples/configs/*.yaml. If the change affects an exemplar covered by tests/unit/test_config_v2.py, also update the matching tests/unit/reference_configs/*.yaml so the v1→v2 migration check stays green.

Recipe YAMLs Must Set defaults

Recipe YAMLs under examples/configs/recipes/**/*.yaml must set defaults: <exemplar>.yaml to inherit from one of the exemplar configs in examples/configs/*.yaml. This keeps recipes minimal — they only override what differs from the exemplar.

If a recipe YAML does not have a defaults key, run:

uv run ./tools/config_cli.py minimize <recipe.yaml>

This will minimize the config and assign the appropriate defaults key.

Accessing NotRequired Fields

When accessing a NotRequired field, use an in check or .get(key) / .get(key, None). Never provide a non-None default — that hides behavior and defeats the purpose of making the field optional.

Do:

# .get() with None (not a hidden default)
stop_properly_penalty_coef = cfg.get("stop_properly_penalty_coef", None)

# Truthiness check for optional booleans
if master_config.grpo.get("skip_reference_policy_logprobs_calculation"):
    ...

# Nested NotRequired: check presence at each level explicitly
if "megatron_cfg" in policy_config and policy_config["megatron_cfg"]["enabled"]:
    ...

Don't:

# Hidden boolean default — should come from YAML
disable_ppo_ratio = cfg.get("disable_ppo_ratio", False)

# Hidden non-trivial default — caller has no idea True is the fallback
normalize_rewards = grpo_config.get("normalize_rewards", True)

# Chained .get() with hidden defaults at each level
megatron_enabled = config.get("megatron_cfg", {}).get("enabled", False)

If a NotRequired field is absent, the code should handle that explicitly — not paper over it with a magic default.

Forbidden Patterns

These are forbidden in both v1 and v2 — they all hide defaults at the call site instead of on the schema.

Don't (any schema type):

# Hidden default at the call site — should be centralized on the BaseModel
# field (v2) or in the exemplar YAML (v1)
precision = policy_cfg.get("precision", "bfloat16")

# Function parameter defaulting a config value
def build_policy(policy_cfg, precision: str = "bfloat16"):
    ...

Do — v2 (BaseModel):

class PolicyConfig(BaseModel, extra="allow"):
    # The BaseModel field is the centralized source of truth for the default
    precision: str = "bfloat16"

# Call site reads the attribute directly
precision = master_config.policy.precision

Do — v1 (TypedDict, still dict-shaped after omegaconf load):

# Required attribute: expect it from the (exemplar or user) YAML, no fallback
precision: str = policy_cfg["precision"]

# Optional (NotRequired) attribute: check for presence, never invent a default
if "milestones" in scheduler_cfg:
    configure_milestones(scheduler_cfg["milestones"])

Avoid dict[str, Any] for Known-Field Config

If a config block has a known set of fields, model it as a BaseModel (or, pre-migration, a TypedDict) — do not type it as dict[str, Any] and read keys with .get(k, default). A bare dict[str, Any] both loses type-safety and pushes per-key defaults to the call site (the Forbidden Pattern above).

If the block must also pass arbitrary keys through to another system, use BaseModel(extra="allow"): the known fields get types + centralized defaults, and unknown keys still come through (model_extra).

Don't:

class NonColocatedTeachersConfig(BaseModel, extra="allow"):
    default_teacher_cfg: dict[str, Any] = Field(default_factory=dict)   # known fields hidden behind Any

# ...so defaults end up scattered at the call site:
tp = cfg.get("tensor_model_parallel_size", 1)
precision = cfg.get("precision", "bf16")

Do:

class TeacherResourceConfig(BaseModel, extra="allow"):  # extra="allow" keeps the passthrough escape hatch
    tensor_model_parallel_size: int = 1
    precision: str = "bf16"
    micro_batch_size: int = 4
    # ...

class NonColocatedTeachersConfig(BaseModel, extra="allow"):
    default_teacher_cfg: TeacherResourceConfig = Field(default_factory=TeacherResourceConfig)

See also: @docs/design-docs/design-and-philosophy.md (Configuration Schema: BaseModel, dataclass, and TypedDict section).