Tidy up lookup_class

rest_framework/serializers.py

@@ -317,17 +317,17 @@ class ModelSerializerOptions(object):
         self.depth = getattr(meta, 'depth', 0)
 
 
-def lookup_class(mapping, obj):
+def lookup_class(mapping, instance):
     """
     Takes a dictionary with classes as keys, and an object.
     Traverses the object's inheritance hierarchy in method
     resolution order, and returns the first matching value
-    from the dictionary or None.
+    from the dictionary or raises a KeyError if nothing matches.
     """
-    return next(
-        (mapping[cls] for cls in inspect.getmro(obj.__class__) if cls in mapping),
-        None
-    )
+    for cls in inspect.getmro(instance.__class__):
+        if cls in mapping:
+            return mapping[cls]
+    raise KeyError('Class %s not found in lookup.', cls.__name__)
 
 
 class ModelSerializer(Serializer):
@@ -341,6 +341,7 @@ class ModelSerializer(Serializer):
         models.DateTimeField: DateTimeField,
         models.DecimalField: DecimalField,
         models.EmailField: EmailField,
+        models.Field: ModelField,
         models.FileField: FileField,
         models.FloatField: FloatField,
         models.ImageField: ImageField,
@@ -484,6 +485,7 @@ class ModelSerializer(Serializer):
         """
         Creates a default instance of a basic non-relational field.
         """
+        serializer_cls = lookup_class(self.field_mapping, model_field)
         kwargs = {}
         validator_kwarg = model_field.validators
 
@@ -602,11 +604,10 @@ class ModelSerializer(Serializer):
         if validator_kwarg:
             kwargs['validators'] = validator_kwarg
 
-        cls = lookup_class(self.field_mapping, model_field)
-        if cls is None:
-            cls = ModelField
+        if issubclass(serializer_cls, ModelField):
             kwargs['model_field'] = model_field
-        return cls(**kwargs)
+
+        return serializer_cls(**kwargs)
 
 
 class HyperlinkedModelSerializerOptions(ModelSerializerOptions):