What is the right way to call `set_defaults`

[aucampia]

The documentation for argparse [ref] suggests using set_defaults for handling subcommands, but I'm not sure what is the right way to call set_defaults with TAP, it would be great if you can add some examples or documentation for this.

[aucampia]

The alternative is to use dest from add_subparsers, but if I use this and also add a type hint for the dest then Tap thinks it should add an argument for it.

So if I do this for example:

class MainParser(Tap):
  subparser_flag: str
  
  ...
  
  def configure(self):
    ...
    self.add_subparsers(required=True, dest="subparser_flag")
    ...

And run, then I get error: the following arguments are required: --subparser_flag - it would be very helpful if there is some example of how subparser support is supposed to work given both the normal ways of using it with ArgumentParser is broken.

[aucampia]

I also tried with _subparser_flag as follow:

class MainParser(Tap):
  _subparser_flag: str
  
  ...
  
  def configure(self):
    ...
    self.add_subparsers(required=True, dest="_subparser_flag")
    ...

But get the same error.

It seems there is some code that attempts to ignore class variables starting with _:

typed-argument-parser/tap/tap.py

Lines 509 to 522 in eb55a9b

	def _get_class_dict(self) -> Dict[str, Any]:
	"""Returns a dictionary mapping class variable names to values from the class dict."""
	class_dict = self._get_from_self_and_super(
	extract_func=lambda super_class: dict(getattr(super_class, '__dict__', dict()))
	)
	class_dict = {
	var: val
	for var, val in class_dict.items()
	if not (var.startswith('_')
	or callable(val)
	or isinstance(val, staticmethod)
	or isinstance(val, classmethod)
	or isinstance(val, property))
	}

But this does not seem to be working as intended, because these still show up in _get_annotations.

Is this another bug? If so I will raise an issue for it.

[aucampia]

As a workaround I have done this:

class MainParser(Tap):

    subparser_flag: str

    def configure(self):
        self.add_argument(
            # Hack to work around problems with Tap, without this Tap complains
            # that `subparser_flag` is not set even if a subcommand is supplied,
            # for more info see
            # <https://github.com/swansonk14/typed-argument-parser/issues/109>.
            "--subparser_flag",
            action="store",
            # dest="subparser_flag",
            help=argparse.SUPPRESS,
            required=False,
        )
        self.add_subparsers(required=True, dest="subparser_flag", help="sub-command")

[martinjm97]

Hi @aucampia,

Unfortunately, we couldn't quite follow this issue. We realize that there's some concern with our current treatment of subparsers. Would you be able to clarify with some example code and then a description of the desired behavior versus the existing behavior?

Thanks,
JK

[ArielHorwitz]

@aucampia
Came here looking for the same issue. It isn't documented, but I managed to guess the solution:

from tap import Tap

class Foo(Tap):
    def configure(self):
        self.set_defaults(func=lambda: print("foo subcommand"))

class Bar(Tap):
    def configure(self):
        self.set_defaults(func=lambda: print("bar subcommand"))

class MyTap(Tap):
    def configure(self):
        self.add_subparsers()
        self.add_subparser("foo", Foo)
        self.add_subparser("bar", Bar)

args = MyTap().parse_args(["foo"])
args.func()  # prints "foo subcommand"


args = MyTap().parse_args(["bar"])
args.func()  # prints "bar subcommand"

Edit: The issue with this solution is that the lines args.func() produce an error in MyPy:
error: "MyTap" has no attribute "func" [attr-defined]

However, I am assuming the typed solution may be out of scope for Tap since this is essentially inheriting the functionality from argparse.