BoolOption does not allow default=None

[vito-laurenza-zocdoc]

Describe the bug

The doc string for the BoolOption type says:

If you don't provide a default value, this becomes a "tri-bool" where the property will return
None if unset by the user.

This is consistent with the other "scalar" option types, however in practice BoolOption does not adhere to this behavior, while the others do.

Given this example subsystem class:

from pants.option.option_types import BoolOption, StrOption, IntOption, FloatOption
from pants.option.subsystem import Subsystem


class MySubsystem(Subsystem):
    name = "My subsystem"
    options_scope = "my-subsystem"
    help = "My subsystem"

    bool_option = BoolOption(
        help="a boolean",
        default=None,
    )
    int_option = IntOption(
        help="an integer",
        default=None,
    )
    float_option = FloatOption(
        help="a float",
        default=None,
    )
    str_option = StrOption(
        help="a string",
        default=None,
    )

We can see in the following help output that the default (and current) value for the BoolOption is False, not None as documented/expected:

$ pants --no-pantsd help my-subsystem

`my-subsystem` subsystem options
--------------------------------

My subsystem

Activated by zocdoc.pants
Config section: [my-subsystem]

  --[no-]my-subsystem-bool-option
  PANTS_MY_SUBSYSTEM_BOOL_OPTION
  bool_option
      default: False
      current value: False
      a boolean

  --my-subsystem-int-option=<int>
  PANTS_MY_SUBSYSTEM_INT_OPTION
  int_option
      default: None
      current value: None
      an integer

  --my-subsystem-float-option=<float>
  PANTS_MY_SUBSYSTEM_FLOAT_OPTION
  float_option
      default: None
      current value: None
      a float

  --my-subsystem-str-option=<str>
  PANTS_MY_SUBSYSTEM_STR_OPTION
  str_option
      default: None
      current value: None
      a string

Pants version

$ pants --version
2.19.0

OS
MacOS on Apple silicon

[vito-laurenza-zocdoc]

Slack discussion: https://pantsbuild.slack.com/archives/C046T6T9U/p1713281183584689

[sureshjoshi]

As per my comment in Slack, apparently you need to use UnsetBool

from pants.option.custom_types import UnsetBool

    foo = BoolOption(
        default=UnsetBool,
        help="bar",
    )