enum with unknown or missing value

enumWithUnknownOrMissing.py

from enum import Enum

class Foo(Enum):
    A = 1
    B = 2
    C = 3

    def __str__(self):
        return self.name

    @classmethod
    def _missing_(cls, value):
        if cls._member_type_ is object:
            # construct a new singleton enum member
            new_member = object.__new__(cls)
            # include the value representation in _name_ because the Enum hash implementation relies on _name_
            new_member._name_ = '%r' % value
            new_member._value_ = value
            # use setdefault in case another thread already created a composite with this value
            new_member = cls._value2member_map_.setdefault(value, new_member)
            return new_member