Default help formatter is not working

[assafge]

example code:
from dataclasses import dataclass, field
from argparse_dataclass import ArgumentParser
from argparse import ArgumentDefaultsHelpFormatter

@dataclass
class FineParams:
    batch_size: int = field(default=300) 

parser = ArgumentParser(GCPFineParams, formatter_class=ArgumentDefaultsHelpFormatter)
params: FineParams = parser.parse_args()

resulting non-defaults help printout.

diving into argparse code (argparse.py, ver 1.1, line 1861) shone that the ArgumentParser's _actions class members, batch_size action is stored as _StoreAction(option_strings=['--batch-size'], dest='batch_size', nargs=None, const=None, default=<dataclasses._MISSING_TYPE object at 0x7f8465889960>, type=<class 'int'>, choices=None, required=False, help=None, metavar=None)

[ymd-h]

I also have same problem, and I want to use default value on help message, too.

Cause

As far as I understand argparse_dataclass explicitly pass MISSING for non-required argument;

argparse_dataclass/argparse_dataclass.py

Line 393 in 540640f

kwargs["default"] = MISSING

This was introduced by this commit to avoid calling default_factory() unnecessarily.

(Possible) Solution

I think we can still pass default to ArgumentParser, and keep to use MISSING for default_factory() case.

Fortunately, only one of the default and default_factory can be specified (ref), I think we can set kwargs["default"] = field.default unconditionally.

What do you think?