Layer

Deterministic, multi-source configuration with validation, provenance tracking, and hot-reloading.

Most applications pull config from several places — a base YAML file, environment variable overrides, a secrets manager in production. Layer gives you a typed pipeline that merges all of them in a defined order, lets you ask where did this value come from? at any time, and validates with different rule sets per environment. The result is a frozen, thread-safe object with no surprises.

Highlights

Feature	Description
Multi-source merging	Add providers in priority order — files, env vars, SSM, Vault, or anything custom
Provenance tracking	Every field records its full source history; source_of() and explain() make debugging instant
Categorical validation	Run different rule sets per environment (prod vs dev) without branching logic
Variable interpolation	${field_name} and ${nested.path} resolved across the full config tree
Type coercion	str → int/bool/float/List[T]/Dict[K,V], Optional[T], Union, Literal — from env vars or files
Parsers	Pre-coercion transforms per field (strip whitespace, remove formatting characters, normalize paths)
Computed fields	Read-only derived properties included in to_dict() and explain()
Hot reloading	Live config updates without restart; reloadable=False locks critical fields
Secret redaction	secret=True fields are automatically masked in explain() and logs
Aliases	Map external key names (e.g. apiKey) to your Python field names
Export artifacts	.env templates, Kubernetes ConfigMaps, JSON Schema
Freeze & thread safety	freeze() locks the config object for safe concurrent reads

Quick Example

from layer import layerclass, field, ConfigPipeline, require, is_port
from layer.providers import FileProvider, EnvProvider

@layerclass
class AppConfig:
    host: str = field(str, default="localhost", description="Service hostname")
    port: int = field(int, default=8080, prod=[require, is_port])
    db_password: str = field(str, default=None, secret=True)

pipeline = (
    ConfigPipeline(AppConfig)
    .add_provider(FileProvider("config.yml"))
    .add_provider(EnvProvider("APP"))      # APP_HOST, APP_PORT, APP_DB_PASSWORD
)

config = pipeline.load()
pipeline.validate(["prod"]).raise_if_invalid()

print(config.host)
print(config.source_of("host"))   # "env:APP_HOST" or "config.yml" or "default"

See Getting Started for the full walkthrough.