I really like this idea! I have very often had to create my own MISSING global in modules where None might actually be a legitimate value for the dataclass member.
And additionally when I am doing thorough type hinting, I haved at times ended up not only have to do the MISSING object, but a MissingType to go with it. Having both of these already present and handled appropriately in the dataclasses library would be a big win, IMO. --- Ricky. "I've never met a Kentucky man who wasn't either thinking about going home or actually going home." - Happy Chandler On Thu, Dec 17, 2020 at 11:56 AM Eelke van den Bos <eelkevd...@gmail.com> wrote: > Hi all, > > Dataclasses provide a unique way of creating parameter bags with decent > typing and validation options. However, they currently lack a meaningful > way of detecting a missing value as a dataclass interface consumer. > > Current behavior allows us to specify defaults per property, but defining > a default value has consequences for the reusability of the dataclass, > especially when it comes to inheritance patterns. Aside from this effect, > littering the code with `str_field: str = MISSING_STR` does not really > provide a clean and readable interface. > > Therefore, I would like to propose an extra option on both the > dataclasses.dataclass decorator as well as a dataclasses.field boolean > argument called `allow_missing`. > > Example: > > @dataclass(allow_missing=True) > class UserBag: > user_id: UUID > > bag = UserBag() # no exception with regards to missing args > bag.user_id # type: Missing[UUID] ? > > if not bag.user_id: # sentinel value evaluates to false > ... > if dataclasses.ismissing(bag.user_id): # or more explicit > ... > > Alternatively, as api on a per-field-basis: > > @dataclass > class UserBag: > user_id: UUID = field(allow_missing=True) > > As a bonus, one would imagine the `asdict` method has an argument to > convert a dataclass instance to a dictionary without the missing properties > (example: `asdict(d, exclude_missing=True)`). > > Generation of code could stay largely the same as it currently is, with > only minor differences: > - type hinting of constructor args should provide a sentinel default value > that adheres to the typing of the original field, something along the lines > of `__init__(self, arg = Missing[str])` > - exception currently raised if a Missing[T] value is detected after > construction should be silenced if the field is marked as missable. > - current sentinel MISSING value evaluates to True when cast as boolean > > I would love to have your feedback on this. > > Best, > > Eelke van den Bos > _______________________________________________ > 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/LG4OXW4AFTOMEXP6SCN6CGDMU6D7RAQ4/ > Code of Conduct: http://python.org/psf/codeofconduct/ >
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