optmanager: Inherit options from parent argument parsers

Allow for including options from parent `argparse.ArgumentParser`
objects in preparation of splitting out the handling of preliminary
options from the `OptionManager`.

src/flake8/options/manager.py

@@ -338,8 +338,12 @@ class OptionManager(object):
     """Manage Options and OptionParser while adding post-processing."""
 
     def __init__(
-        self, prog, version, usage="%(prog)s [options] file file ..."
-    ):  # type: (str, str, str) -> None
+        self,
+        prog,
+        version,
+        usage="%(prog)s [options] file file ...",
+        parents=None,
+    ):  # type: (str, str, str, Optional[List[argparse.ArgumentParser]]) -> None  # noqa: E501
         """Initialize an instance of an OptionManager.
 
         :param str prog:
@@ -348,9 +352,15 @@ class OptionManager(object):
             Version string for the program.
         :param str usage:
             Basic usage string used by the OptionParser.
+        :param argparse.ArgumentParser parents:
+            A list of ArgumentParser objects whose arguments should also be
+            included.
         """
+        if parents is None:
+            parents = []
+
         self.parser = argparse.ArgumentParser(
-            prog=prog, usage=usage
+            prog=prog, usage=usage, parents=parents
         )  # type: argparse.ArgumentParser
         self._current_group = None  # type: Optional[argparse._ArgumentGroup]
         self.version_action = cast(

tests/unit/test_option_manager.py

@@ -22,6 +22,23 @@ def test_option_manager_creates_option_parser(optmanager):
     assert isinstance(optmanager.parser, argparse.ArgumentParser)
 
 
+def test_option_manager_including_parent_options():
+    """Verify parent options are included in the parsed options."""
+    # GIVEN
+    parent_parser = argparse.ArgumentParser(add_help=False)
+    parent_parser.add_argument('--parent')
+
+    # WHEN
+    optmanager = manager.OptionManager(
+        prog='flake8',
+        version=TEST_VERSION,
+        parents=[parent_parser])
+    option, _ = optmanager.parse_args(['--parent', 'foo'])
+
+    # THEN
+    assert option.parent == 'foo'
+
+
 def test_parse_args_forwarding_default_values(optmanager):
     """Verify default provided values are present in the final result."""
     namespace = argparse.Namespace(foo='bar')