[Python-ideas] Re: factory for sentinel objects
There is already https://pypi.org/project/sentinel/ https://pypi.org/project/sentinels/ Though, I think this should become part of the standard library. It's a fundamental concept, somewhat analogous to namedtuples, enums and dataclass (only a bit less used, but also less complex). Once figured out, the implementation will be very stable. I believe it's not good to depend for such small and stable functionality on third party packages. At least, I'd often then just stay with the sentinel = object() approach. I think https://peps.python.org/pep-0661/ is going very much in the right direction. Will continue discussing in this context. > Matthias Görgens hat am 31.08.2023 11:29 CEST > geschrieben: > > > Seems nice. Just write a library and upload it to one of the usual places? > ___ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-le...@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/4L7EU26NWJGUZPZ3W5WXRGQJLKNRSLDH/ Code of Conduct: http://python.org/psf/codeofconduct/
[Python-ideas] Re: factory for sentinel objects
Seems nice. Just write a library and upload it to one of the usual places?
On Thu, 31 Aug 2023, 16:54 Tim Hoffmann via Python-ideas, <
python-ideas@python.org> wrote:
> 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:
>
> - 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)
>
> >>> 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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[Python-ideas] Re: factory for sentinel objects
On Thu, 31 Aug 2023 at 18:55, Tim Hoffmann via Python-ideas wrote: > > 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: > Looks like you may be thinking of this: https://peps.python.org/pep-0661/ There are a few options here, including classes, enums, and a dedicated sentinel type. ChrisA ___ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-le...@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/6C2FSMYQ2PBRUXRU7MVXTN2J6OJH64NZ/ Code of Conduct: http://python.org/psf/codeofconduct/
[Python-ideas] factory for sentinel objects
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:
- 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)
>>> 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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[Python-ideas] Re: factory for sentinel objects
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 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:
>
> - 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)
>
> >>> 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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