Allow type to be default arg (#558)

* Add test case for type defined using default_args

* Refactor build_from_cfg

* Update exception of missing type

* pre-commit

* Fix default_args is None

* pre-commit

* Bring back test

* Update exception raising

mmcv/utils/registry.py

@@ -139,8 +139,10 @@ def build_from_cfg(cfg, registry, default_args=None):
     if not isinstance(cfg, dict):
         raise TypeError(f'cfg must be a dict, but got {type(cfg)}')
     if 'type' not in cfg:
-        raise KeyError(
-            f'the cfg dict must contain the key "type", but got {cfg}')
+        if default_args is None or 'type' not in default_args:
+            raise KeyError(
+                '`cfg` or `default_args` must contain the key "type", '
+                f'but got {cfg}\n{default_args}')
     if not isinstance(registry, Registry):
         raise TypeError('registry must be an mmcv.Registry object, '
                         f'but got {type(registry)}')
@@ -149,6 +151,11 @@ def build_from_cfg(cfg, registry, default_args=None):
                         f'but got {type(default_args)}')
 
     args = cfg.copy()
+
+    if default_args is not None:
+        for name, value in default_args.items():
+            args.setdefault(name, value)
+
     obj_type = args.pop('type')
     if is_str(obj_type):
         obj_cls = registry.get(obj_type)
@@ -161,7 +168,4 @@ def build_from_cfg(cfg, registry, default_args=None):
         raise TypeError(
             f'type must be a str or valid type, but got {type(obj_type)}')
 
-    if default_args is not None:
-        for name, value in default_args.items():
-            args.setdefault(name, value)
     return obj_cls(**args)

tests/test_utils/test_registry.py

@@ -158,6 +158,18 @@ def test_build_from_cfg():
     assert isinstance(model, ResNet)
     assert model.depth == 50 and model.stages == 4
 
+    # type defined using default_args
+    cfg = dict(depth=50)
+    model = mmcv.build_from_cfg(
+        cfg, BACKBONES, default_args=dict(type='ResNet'))
+    assert isinstance(model, ResNet)
+    assert model.depth == 50 and model.stages == 4
+
+    cfg = dict(depth=50)
+    model = mmcv.build_from_cfg(cfg, BACKBONES, default_args=dict(type=ResNet))
+    assert isinstance(model, ResNet)
+    assert model.depth == 50 and model.stages == 4
+
     # not a registry
     with pytest.raises(TypeError):
         cfg = dict(type='VGG')
@@ -179,10 +191,16 @@ def test_build_from_cfg():
         model = mmcv.build_from_cfg(cfg, BACKBONES)
 
     # cfg should contain the key "type"
-    with pytest.raises(KeyError):
+    with pytest.raises(KeyError, match='must contain the key "type"'):
         cfg = dict(depth=50, stages=4)
         model = mmcv.build_from_cfg(cfg, BACKBONES)
 
+    # cfg or default_args should contain the key "type"
+    with pytest.raises(KeyError, match='must contain the key "type"'):
+        cfg = dict(depth=50)
+        model = mmcv.build_from_cfg(
+            cfg, BACKBONES, default_args=dict(stages=4))
+
     # incorrect registry type
     with pytest.raises(TypeError):
         cfg = dict(type='ResNet', depth=50)