"""

    >>> class Missing(Sentinel): pass
    >>> Missing
    Missing
    >>> class CustomRepr(Sentinel):
    ...     repr = '<CustomRepr>'
    ...
    >>> CustomRepr
    <CustomRepr>

"""

class SentinelMeta(type):
    def __repr__(cls):
        try:
            return cls.repr
        except AttributeError:
            return cls.__name__

class Sentinel(metaclass=SentinelMeta):
    def __new__(cls):
        return cls