There are four cases which I think should be covered with validator instance arguments:

• Allow known missed, forbid unknown present (default behavior right now)
• Allow known missed, allow unknown present (allow_unknown=True)
• Forbid known missed, forbid unknown present (no way at validator creation time)
• Forbid known missed, allow unkown present (no way at validator creation time)

Cerberus have allow_unknown option for validator, but have no allow_known_missed / forbid_known_missed or something like this. As stackoverflow question said, it is pretty annoying to set required: True for all fields, cause they can be nested and schema looks noisy and dirty.