The standard pattern to create a sentinel in Python is

>>> Unset = object()

While this is often good enough, it has some shortcomings:

- repr(Unset) is unhelpful: <object at 0x1ded9911b60>

- copy/deepcopy create a copy of the sentinel object, which can lead to surprising results such as:

>>> d = {'val': Unset}

>>> d['val'] is Unset

True

>>> d2 = copy.deepcopy(d)

>>> d2['val'] is Unset

False

- The code "Unset = object()" is quite obscure for people not familiar with the sentinel pattern.

I propose to provide a sentinel() factory function so that

>>> Unset = sentinel("Unset")

>>> repr(unset)

<Unset>

>>> copy.copy(Unset) is Unset

True

>>> copy.deepcopy(Unset) is Unset

True

A simple implementation would be something like

def sentinel(name):
    return type(name, (), {
        '__repr__': lambda self: f"<{self.__class__.__name__}>",
        '__copy__': lambda self: self,
        '__deepcopy__': lambda self, memo: self,
    })

Likely, the implementation should be refined a bit more.

While it's not rocket science, getting sentinels correct is cumbersome for end users. Providing such a function in the standard library is only a minor maintainance burden, but a significant help for users working with sentinels.

Thanks for your consideration!

Tim