[Fix] fix config type inconsistency (#1575)

* [Fix] fix config type inconsistency

* [Fix] Fix unit test

mmcv/utils/config.py

@@ -310,16 +310,19 @@ class Config:
                 if len(b) <= k:
                     raise KeyError(f'Index {k} exceeds the length of list {b}')
                 b[k] = Config._merge_a_into_b(v, b[k], allow_list_keys)
-            elif isinstance(v,
-                            dict) and k in b and not v.pop(DELETE_KEY, False):
+            elif isinstance(v, dict):
+                if k in b and not v.pop(DELETE_KEY, False):
                     allowed_types = (dict, list) if allow_list_keys else dict
                     if not isinstance(b[k], allowed_types):
                         raise TypeError(
-                        f'{k}={v} in child config cannot inherit from base '
-                        f'because {k} is a dict in the child config but is of '
-                        f'type {type(b[k])} in base config. You may set '
-                        f'`{DELETE_KEY}=True` to ignore the base config')
+                            f'{k}={v} in child config cannot inherit from '
+                            f'base because {k} is a dict in the child config '
+                            f'but is of type {type(b[k])} in base config. '
+                            f'You may set `{DELETE_KEY}=True` to ignore the '
+                            f'base config.')
                     b[k] = Config._merge_a_into_b(v, b[k], allow_list_keys)
+                else:
+                    b[k] = ConfigDict(v)
             else:
                 b[k] = v
         return b

tests/data/config/delete.py

 _base_ = './base.py'
-item2 = {'b': 0, '_delete_': True}
+item1 = {'a': 0, '_delete_': True}
+item2 = {'b': 0}

tests/test_utils/test_config.py

@@ -10,7 +10,7 @@ from pathlib import Path
 import pytest
 import yaml
 
-from mmcv import Config, DictAction, dump, load
+from mmcv import Config, ConfigDict, DictAction, dump, load
 
 data_path = osp.join(osp.dirname(osp.dirname(__file__)), 'data')
 
@@ -347,12 +347,16 @@ def test_merge_delete():
     cfg_file = osp.join(data_path, 'config/delete.py')
     cfg = Config.fromfile(cfg_file)
     # cfg.field
-    assert cfg.item1 == [1, 2]
-    assert cfg.item2 == dict(b=0)
+    assert cfg.item1 == dict(a=0)
+    assert cfg.item2 == dict(a=0, b=0)
     assert cfg.item3 is True
     assert cfg.item4 == 'test'
     assert '_delete_' not in cfg.item2
 
+    # related issue: https://github.com/open-mmlab/mmcv/issues/1570
+    assert type(cfg.item1) == ConfigDict
+    assert type(cfg.item2) == ConfigDict
+
 
 def test_merge_intermediate_variable():