Fix get_schema to preserve the definition order

although it will no longer pick up fields of superclasses.

mkdocs/config/base.py

@@ -207,8 +207,7 @@ def get_schema(cls: type) -> PlainConfigSchema:
 
     See mkdocs/config/defaults.py for an example.
     """
-    all_items = ((k, getattr(cls, k)) for k in dir(cls) if not k.startswith('_'))
-    return tuple((k, v) for k, v in all_items if isinstance(v, BaseConfigOption))
+    return tuple((k, v) for k, v in cls.__dict__.items() if isinstance(v, BaseConfigOption))
 
 
 @contextmanager

mkdocs/tests/config/base_tests.py

@@ -2,7 +2,7 @@ import os
 import unittest
 
 from mkdocs import exceptions
-from mkdocs.config import base, defaults
+from mkdocs.config import base, config_options, defaults
 from mkdocs.config.base import ValidationError
 from mkdocs.config.config_options import BaseConfigOption
 from mkdocs.tests.base import change_dir, tempdir
@@ -268,3 +268,16 @@ class ConfigBaseTests(unittest.TestCase):
         self.assertEqual(cfg['docs_dir'], docs_dir)
         self.assertEqual(cfg.config_file_path, config_fname)
         self.assertIsInstance(cfg.config_file_path, str)
+
+    def test_get_schema(self):
+        class FooConfig:
+            z = config_options.URL()
+            aa = config_options.Type(int)
+
+        self.assertEqual(
+            base.get_schema(FooConfig),
+            (
+                ('z', FooConfig.z),
+                ('aa', FooConfig.aa),
+            ),
+        )