Improve field lookup behavior for dicts/mappings. Closes #2244. Closes …

…#2243.

rest_framework/fields.py

@@ -15,6 +15,7 @@
 from rest_framework.exceptions import ValidationError
 from rest_framework.settings import api_settings
 from rest_framework.utils import html, representation, humanize_datetime
+import collections
 import copy
 import datetime
 import decimal
@@ -60,14 +61,12 @@ def get_attribute(instance, attrs):
             # Break out early if we get `None` at any point in a nested lookup.
             return None
         try:
-            instance = getattr(instance, attr)
+            if isinstance(instance, collections.Mapping):
+                instance = instance[attr]
+            else:
+                instance = getattr(instance, attr)
         except ObjectDoesNotExist:
             return None
-        except AttributeError as exc:
-            try:
-                return instance[attr]
-            except (KeyError, TypeError, AttributeError):
-                raise exc
         if is_simple_callable(instance):
             instance = instance()
     return instance