Sorry, minor bug in the example implementation:
def sentinel(name):
cls = type(name, (), {
'__repr__': lambda self: f"<{self.__class__.__name__}>",
'__copy__': lambda self: self,
'__deepcopy__': lambda self, memo: self,
})
return cls()
> Tim Hoffmann <tim.hoffm...@mailbox.org> hat am 31.08.2023 10:44 CEST
> geschrieben:
>
>
> 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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