Mandatory missing values

Use ??? to mark a mandatory missing 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

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

In OmegaConf 2.3, the string ??? is always interpreted as missing when stored in a config, even if it was quoted in YAML. The 2.4 documentation describes the new escape form for literal text.

Next, learn how merging config sources treats missing values.