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_______________________________________________
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