Missing values

Use ??? for a value that must be supplied later. Reading it raises MissingMandatoryValue; OmegaConf.is_missing() lets you check it without reading it:

>>> from omegaconf import OmegaConf
>>> cfg = OmegaConf.create({
...     "host": "???",
...     "port": 80,
... })
>>> OmegaConf.is_missing(cfg, "host")
True
>>> cfg.host
Traceback (most recent call last):
...
omegaconf.errors.MissingMandatoryValue: Missing mandatory value: host
    full_key: host
    object_type=dict

MISSING is the Python spelling of the same marker. None is an ordinary value, not a missing marker.

Literal ??? text

In 2.4, prefix ??? with a backslash to store those three characters as literal text. YAML quoting alone does not change the missing marker:

>>> cfg = OmegaConf.create({
...     "required": "???",
...     "literal": r"\???",
... })
>>> OmegaConf.is_missing(cfg, "required")
True
>>> cfg.literal == "???"
True
>>> OmegaConf.is_missing(cfg, "literal")
False

The literal value retains a marker internally so it remains literal through interpolation and serialization. When checking a detached value, use OmegaConf.is_missing(value) instead of comparing it with the string ???. For a literal backslash followed by ???, write \\??? in the config input.

Next, learn how merging config sources treats missing values.