marsh.MISSING

This value is treated as the unset value by marsh (not to be confused with None which may be a valid value).

This can be useful when a default value is required by python even though the programmer wants the value to be required (missing).

import dataclasses
import marsh

@dataclasses.dataclass
class Config:
    a: int = 3

@dataclasses.dataclass
class ExpandedConfig(Config):
    b: str

The code above will fail to run, throwing TypeError: non-default argument 'b' follows default argument

Instead we can use marsh.MISSING

...

@dataclasses.dataclass
class ExpandedConfig(Config):
    b: str  = marsh.MISSING

Trying to unmarshal this code we get the correct behavior

...

# fails with marsh.errors.MissingValueError
marsh.unmarshal(ExpandedConfig, {'a': 2})

Note

Using marsh.MISSING as input when unmarshaling is sometimes allowed. For example, None accepts marsh.MISSING. Mappings and sequences will also default to an empty instance when the input is marsh.MISSING.

The value of marsh.MISSING is the same as omegaconf.MISSING, represented by the string '???'.