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