MultipleChoiceField empties incorrectly on a partial update using multipart/form-data (#2894)

rest_framework/fields.py

@@ -1060,7 +1060,11 @@ def get_value(self, dictionary):
         # We override the default field access in order to support
         # lists in HTML forms.
         if html.is_html_input(dictionary):
-            return dictionary.getlist(self.field_name)
+            ret = dictionary.getlist(self.field_name)
+            if getattr(self.root, 'partial', False) and not ret:
+                ret = empty
+            return ret
+
         return dictionary.get(self.field_name, empty)
 
     def to_internal_value(self, data):

tests/test_fields.py

@@ -1,6 +1,7 @@
 from decimal import Decimal
 from django.utils import timezone
 from rest_framework import serializers
+import rest_framework
 import datetime
 import django
 import pytest
@@ -1017,6 +1018,15 @@ class TestMultipleChoiceField(FieldValues):
         ]
     )
 
+    def test_against_partial_and_full_updates(self):
+        # serializer = self.Serializer(data=MockHTMLDict())
+        from django.http import QueryDict
+        field = serializers.MultipleChoiceField(choices=(('a', 'a'), ('b', 'b')))
+        field.partial = False
+        assert field.get_value(QueryDict({})) == []
+        field.partial = True
+        assert field.get_value(QueryDict({})) == rest_framework.fields.empty
+
 
 # File serializers...