[issue44123] make function parameter sentinel value true singletons

Tal Einat added the comment: > And what you need is just to not pass to the wrapped function arguments which > were not passed to wrapper. *args and **kwargs do not contain arguments which > were not passed. It's common for wrapper functions to mirror the defaults of the wrapped functions

[issue44123] make function parameter sentinel value true singletons

Serhiy Storchaka added the comment: Parameters are not passed to a function. Arguments are passed. And what you need is just to not pass to the wrapped function arguments which were not passed to wrapper. *args and **kwargs do not contain arguments which were not passed. I understand that

[issue44123] make function parameter sentinel value true singletons

Tal Einat added the comment: > I do not understand the problem with pickling sentinel values used as default > values for function parameters. Could you please show an example? A category of relevant uses I can think of is when wrapping a function and storing the parameters it was called

[issue44123] make function parameter sentinel value true singletons

STINNER Victor added the comment: Tal: "This is also being discussed on python-dev." https://mail.python.org/archives/list/python-...@python.org/thread/ZLVPD2OISI7M4POMTR2FCQTE6TPMPTO3/ -- nosy: +vstinner ___ Python tracker

[issue44123] make function parameter sentinel value true singletons

Tal Einat added the comment: > This needs a good deal more discussion before sweeping through the code and > change a long standing Python idiom that has stood the test of time. I agree this should be discussed, which is why I created this issue about it, as place for that discussion to

[issue44123] make function parameter sentinel value true singletons

Serhiy Storchaka added the comment: I do not understand the problem with pickling sentinel values used as default values for function parameters. Could you please show an example? -- nosy: +serhiy.storchaka ___ Python tracker

[issue44123] make function parameter sentinel value true singletons

Raymond Hettinger added the comment: This needs a good deal more discussion before sweeping through the code and change a long standing Python idiom that has stood the test of time. Until now, no one has claimed that this is broken. You "recently noticing this" doesn't mean that it is

[issue44123] make function parameter sentinel value true singletons

Tal Einat added the comment: ... and they can be given excellent reprs by using a meta-class: class Sentinel(type): @classmethod def __prepare__(cls, name, bases, **kwds): d = super().__prepare__(name, bases, **kwds) def __new__(cls_, *args, **kwargs):

[issue44123] make function parameter sentinel value true singletons

Tal Einat added the comment: Alternatively, sentinels can simply be classes: class Sentinel: def __new__(cls, *args, **kwargs): raise TypeError(f'{cls.__qualname__} cannot be instantiated') class MISSING(Sentinel): pass -- ___

[issue44123] make function parameter sentinel value true singletons

Tal Einat added the comment: Here is what I suggest working with sentinels would look like: >>> from dataclasses import MISSING >>> MISSING dataclasses.MISSING >>> M2 = pickle.loads(pickle.dumps(MISSING)) >>> M2 dataclasses.MISSING >>> M2 is MISSING True Here's an implementation which

[issue44123] make function parameter sentinel value true singletons

Change by Eric V. Smith : -- nosy: +eric.smith ___ Python tracker ___ ___ Python-bugs-list mailing list Unsubscribe:

[issue44123] make function parameter sentinel value true singletons

Zachary Ware added the comment: FWIW, at work we've been using `...` as a handy not-None singleton. -- nosy: +zach.ware ___ Python tracker ___

[issue44123] make function parameter sentinel value true singletons

Tal Einat added the comment: Additional examples of such sentinel values in the stdlib are those in the dataclasses module, e.g. dataclasses.MISSING. -- ___ Python tracker

[issue44123] make function parameter sentinel value true singletons

New submission from Tal Einat : I recently noticed that some functions use sentinel values to differentiate between not having been passed any value vs. None. One example of this is traceback.print_exception(), hence I'm adding Irit to the nosy list. However, using e.g. `sentinel = object()`