Not

The not_ keyword is a logical operator.

The return value will be True if the statement(s) are not True, otherwise it will return False.

from rules_engine import Rule, RulesEngine, not_

def is_missing(obj):
    return not obj

obj="Hello"

RulesEngine(Rule(not_(is_missing), then(True)), 'object is missing').run(obj)

>>> Result(value=True, message='object is missing')