fix: MultipleChoiceField use ordered sort

docs/api-guide/fields.md

@@ -417,7 +417,7 @@ Both the `allow_blank` and `allow_null` are valid options on `ChoiceField`, alth
 
 ## MultipleChoiceField
 
-A field that can accept a set of zero, one or many values, chosen from a limited set of choices. Takes a single mandatory argument. `to_internal_value` returns a `set` containing the selected values.
+A field that can accept a list of zero, one or many values, chosen from a limited set of choices. Takes a single mandatory argument. `to_internal_value` returns a `list` containing the selected values, deduplicated
 
 **Signature:** `MultipleChoiceField(choices)`
 

rest_framework/fields.py

@@ -1523,17 +1523,17 @@ def to_internal_value(self, data):
         if not self.allow_empty and len(data) == 0:
             self.fail('empty')
 
-        return {
-            # Arguments for super() are needed because of scoping inside
-            # comprehensions.
+        # Arguments for super() are needed because of scoping inside
+        # comprehensions.
+        return list(dict.fromkeys([
             super(MultipleChoiceField, self).to_internal_value(item)
             for item in data
-        }
+        ]))
 
     def to_representation(self, value):
-        return {
+        return list(dict.fromkeys([
             self.choice_strings_to_values.get(str(item), item) for item in value
-        }
+        ]))
 
 
 class FilePathField(ChoiceField):

tests/test_fields.py

@@ -24,6 +24,7 @@
     BuiltinSignatureError, DjangoImageField, SkipField, empty,
     is_simple_callable
 )
+from rest_framework.utils import json
 from tests.models import UUIDForeignKeyTarget
 
 utc = datetime.timezone.utc
@@ -2178,16 +2179,20 @@ class TestMultipleChoiceField(FieldValues):
     Valid and invalid values for `MultipleChoiceField`.
     """
     valid_inputs = {
-        (): set(),
-        ('aircon',): {'aircon'},
-        ('aircon', 'manual'): {'aircon', 'manual'},
+        (): list(),
+        ('aircon',): ['aircon'],
+        ('aircon', 'aircon'): ['aircon'],
+        ('aircon', 'manual'): ['aircon', 'manual'],
+        ('manual', 'aircon'): ['manual', 'aircon'],
     }
     invalid_inputs = {
         'abc': ['Expected a list of items but got type "str".'],
         ('aircon', 'incorrect'): ['"incorrect" is not a valid choice.']
     }
     outputs = [
-        (['aircon', 'manual', 'incorrect'], {'aircon', 'manual', 'incorrect'})
+        (['aircon', 'manual', 'incorrect'], ['aircon', 'manual', 'incorrect']),
+        (['manual', 'aircon', 'incorrect'], ['manual', 'aircon', 'incorrect']),
+        (['aircon', 'manual', 'aircon'], ['aircon', 'manual']),
     ]
     field = serializers.MultipleChoiceField(
         choices=[
@@ -2204,6 +2209,27 @@ def test_against_partial_and_full_updates(self):
         field.partial = True
         assert field.get_value(QueryDict('')) == rest_framework.fields.empty
 
+    def test_valid_inputs_is_json_serializable(self):
+        for input_value, _ in get_items(self.valid_inputs):
+            validated = self.field.run_validation(input_value)
+
+            try:
+                json.dumps(validated)
+            except TypeError as e:
+                pytest.fail(f'Validated output not JSON serializable: {repr(validated)}; Error: {e}')
+
+    def test_output_is_json_serializable(self):
+        for output_value, _ in get_items(self.outputs):
+            representation = self.field.to_representation(output_value)
+
+            try:
+                json.dumps(representation)
+            except TypeError as e:
+                pytest.fail(
+                    f'to_representation output not JSON serializable: '
+                    f'{repr(representation)}; Error: {e}'
+                )
+
 
 class TestEmptyMultipleChoiceField(FieldValues):
     """

tests/test_serializer_nested.py

@@ -199,14 +199,14 @@ def test_nested_serializer_with_list_json(self):
         serializer = self.Serializer(data=input_data)
 
         assert serializer.is_valid()
-        assert serializer.validated_data['nested']['example'] == {1, 2}
+        assert serializer.validated_data['nested']['example'] == [1, 2]
 
     def test_nested_serializer_with_list_multipart(self):
         input_data = QueryDict('nested.example=1&nested.example=2')
         serializer = self.Serializer(data=input_data)
 
         assert serializer.is_valid()
-        assert serializer.validated_data['nested']['example'] == {1, 2}
+        assert serializer.validated_data['nested']['example'] == [1, 2]
 
 
 class TestNotRequiredNestedSerializerWithMany: