Set configuration file-provided values via ArgumentParser.set_defaults()

When calling `ArgumentParser.parse_args()` with the `namespace`
argument, command-line options are just added to the  namespace without
going through any of the argument parsing and type conversion logic
(e.g., the `type` keyword argument of `ArgumentParser.add_argument()`).
In other words, it is assumed that a namespace is well-formed from a
previous invocation of `ArgumentParser.parse_args()`.

The `values` parameter is intended to be values already-provided from
configuration files.  To take advantage of the logic defined by
`ArgumentParser.add_argument()`,  utilize
`ArgumentParser.set_defaults()` instead.

src/flake8/options/manager.py

@@ -433,7 +433,9 @@ class OptionManager(object):
         assert isinstance(  # nosec (for bandit)
             self.parser, argparse.ArgumentParser
         ), self.parser
-        parsed_args = self.parser.parse_args(args, values)
+        if values:
+            self.parser.set_defaults(**vars(values))
+        parsed_args = self.parser.parse_args(args)
         # TODO: refactor callers to not need this
         return parsed_args, parsed_args.filenames
 

tests/unit/test_option_manager.py

@@ -29,6 +29,14 @@ def test_parse_args_forwarding_default_values(optmanager):
     assert options.foo == 'bar'
 
 
+def test_parse_args_forwarding_type_coercion(optmanager):
+    """Verify default provided values are type converted from add_option."""
+    optmanager.add_option('--foo', type=int)
+    namespace = argparse.Namespace(foo='5')
+    options, args = optmanager.parse_args([], namespace)
+    assert options.foo == 5
+
+
 def test_add_option_short_option_only(optmanager):
     """Verify the behaviour of adding a short-option only."""
     assert optmanager.options == []