@Config#

Load a class’s fields from environment variables, coerced to their annotated types, at construction — then inject the settings object by type.

The problem it solves#

Twelve-factor apps read configuration from the environment, but hand-writing the plumbing is repetitive and error-prone: os.environ["PORT"], remembering to int(...) it, parsing "true"/"1" into a bool, applying a default when the variable is absent, and raising something useful when a required one is missing. @Config derives all of that from the class’s annotations, once.

Usage#

from inito import Config


@Config(prefix="APP_")
class Settings:
    database_url: str                 # required  -> reads APP_DATABASE_URL
    port: int = 5432                  # optional  -> APP_PORT, coerced to int
    debug: bool = False               # optional  -> APP_DEBUG, parsed as a bool


settings = Settings()                 # reads os.environ at construction

Each field maps to PREFIX + FIELD_NAME.upper(). A field with a value in the environment uses it (coerced); a field without one falls back to its default; a required field with neither raises ConfigResolutionError.

Booleans accept 1/true/yes/on (and 0/false/no/off), case-insensitively. str, int, float, and Optional[...] are supported; any other type receives the raw string.

What it generates#

A zero-argument __init__ that reads the environment and assigns each field. The field → variable → coercer → default plan is computed once, at decoration time; only the os.environ lookups happen at construction (which, for a singleton, is once).

Options#

Option	Default	Effect
`prefix`	`""`	prepended to every field’s environment-variable name

Use with dependency injection#

@Config classes are meant to be injected. Register one as a @Service and it is autowired by type into any constructor that asks for it:

from inito import Config, Service


@Service
@Config(prefix="APP_")
class Settings:
    database_url: str = "sqlite:///app.db"


@Service
class Database:
    def __init__(self, settings: Settings) -> None:
        self.url = settings.database_url

See Configuration injection.

Notes & gotchas#

Values are read from os.environ at construction, not at import — set the environment (or monkeypatch.setenv(...) in tests) before the object is built.
A required field with no environment value and no default raises ConfigResolutionError; a value that can’t be coerced (e.g. PORT=abc for an int) raises it too.
Need validation, nested config, or .env files? A Pydantic BaseSettings works with InitO’s DI the same way — register it as a @Service and it autowires by type.