Python class attribute 'is not defined' when referenced by another class attribute

Question

Using the following, I am able to successfully create a parser and add my arguments to self._parser through the __init()__ method.

class Parser:
    _parser_params = {
        'description': 'Generate a version number from the version configuration file.',
        'allow_abbrev': False
    }
    _parser = argparse.ArgumentParser(**_parser_params)

Now I wish to split the arguments into groups so I have updated my module, adding some classes to represent the argument groups (in reality there are several subclasses of the ArgumentGroup class), and updating the Parser class.

class ArgumentGroup:
    _title = None
    _description = None

    def __init__(self, parser) -> ArgumentParser:
        parser.add_argument_group(*self._get_args())

    def _get_args(self) -> list:
        return [self._title, self._description]


class ArgumentGroup_BranchType(ArgumentGroup):
    _title = 'branch type arguments'


class Parser:
    _parser_params = {
        'description': 'Generate a version number from the version configuration file.',
        'allow_abbrev': False
    }
    _parser = argparse.ArgumentParser(**_parser_params)
    _argument_groups = [cls(_parser) for cls in ArgumentGroup.__subclasses__()]

However, I'm now seeing an error.

Traceback (most recent call last):
  ...
  File "version2/args.py", line 62, in <listcomp>
    _argument_groups = [cls(_parser) for cls in ArgumentGroup.__subclasses__()]
NameError: name '_parser' is not defined

What I don't understand is why _parser_params do exist when they are referred by another class attribute, but _parser seemingly does not exist in the same scenario? How can I refactor my code to add the parser groups as required?