Optional fields

A structured config uses a field's type annotation to check assigned values. T | None means that the field accepts either a value of type T or None:

>>> from dataclasses import dataclass
>>> from omegaconf import MISSING, OmegaConf
>>> @dataclass
... class Service:
...     token: str | None = None
...     host: str = "localhost"
>>> cfg = OmegaConf.structured(Service)
>>> cfg.token is None
True
>>> cfg.token = "secret"
>>> cfg.token
'secret'
>>> OmegaConf.update(cfg, "host", None)
Traceback (most recent call last):
...
omegaconf.errors.ValidationError: field 'host' is not Optional
    full_key: host
    object_type=Service

Optionality and missing values answer different questions. T | None permits None; MISSING means no value has been supplied yet. An optional field can start out missing and later receive None:

>>> @dataclass
... class Credentials:
...     token: str | None = MISSING
>>> credentials = OmegaConf.structured(Credentials)
>>> OmegaConf.is_missing(credentials, "token")
True
>>> credentials.token = None
>>> OmegaConf.is_missing(credentials, "token")
False

To apply a schema to external data, continue with Validate data with a schema.