Add strict_choices to ChoiceField & MultipleChoiceField

docs/api-guide/fields.md

@@ -404,6 +404,7 @@ Used by `ModelSerializer` to automatically generate fields if the corresponding
 - `allow_blank` - If set to `True` then the empty string should be considered a valid value. If set to `False` then the empty string is considered invalid and will raise a validation error. Defaults to `False`.
 - `html_cutoff` - If set this will be the maximum number of choices that will be displayed by a HTML select drop down. Can be used to ensure that automatically generated ChoiceFields with very large possible selections do not prevent a template from rendering. Defaults to `None`.
 - `html_cutoff_text` - If set this will display a textual indicator if the maximum number of items have been cutoff in an HTML select drop down. Defaults to `"More than {count} items…"`
+- `strict_choices` - If set to `True` then `to_representation()` checks if output is one of choices. Defaults to `False`.
 
 Both the `allow_blank` and `allow_null` are valid options on `ChoiceField`, although it is highly recommended that you only use one and not both. `allow_blank` should be preferred for textual choices, and `allow_null` should be preferred for numeric or other non-textual choices.
 
@@ -417,6 +418,7 @@ A field that can accept a set of zero, one or many values, chosen from a limited
 - `allow_blank` - If set to `True` then the empty string should be considered a valid value. If set to `False` then the empty string is considered invalid and will raise a validation error. Defaults to `False`.
 - `html_cutoff` - If set this will be the maximum number of choices that will be displayed by a HTML select drop down. Can be used to ensure that automatically generated ChoiceFields with very large possible selections do not prevent a template from rendering. Defaults to `None`.
 - `html_cutoff_text` - If set this will display a textual indicator if the maximum number of items have been cutoff in an HTML select drop down. Defaults to `"More than {count} items…"`
+- `strict_choices` - If set to `True` then `to_representation()` checks if output is a set from choices. Defaults to `False`.
 
 As with `ChoiceField`, both the `allow_blank` and `allow_null` options are valid, although it is highly recommended that you only use one and not both. `allow_blank` should be preferred for textual choices, and `allow_null` should be preferred for numeric or other non-textual choices.
 

rest_framework/fields.py

@@ -1416,6 +1416,7 @@ class ChoiceField(Field):
         self.html_cutoff_text = kwargs.pop('html_cutoff_text', self.html_cutoff_text)
 
         self.allow_blank = kwargs.pop('allow_blank', False)
+        self.strict_choices = kwargs.pop('strict_choices', False)
 
         super().__init__(**kwargs)
 
@@ -1429,6 +1430,12 @@ class ChoiceField(Field):
             self.fail('invalid_choice', input=data)
 
     def to_representation(self, value):
+        if self.strict_choices:
+            try:
+                return self.choice_strings_to_values[str(value)]
+            except KeyError:
+                self.fail('invalid_choice', input=value)
+
         if value in ('', None):
             return value
         return self.choice_strings_to_values.get(str(value), value)
@@ -1494,6 +1501,17 @@ class MultipleChoiceField(ChoiceField):
         }
 
     def to_representation(self, value):
+        if self.strict_choices:
+            if isinstance(value, str) or not hasattr(value, '__iter__'):
+                self.fail('not_a_list', input_type=type(value).__name__)
+            if not self.allow_empty and len(value) == 0:
+                self.fail('empty')
+
+            return {
+                super(MultipleChoiceField, self).to_representation(item)
+                for item in value
+            }
+
         return {
             self.choice_strings_to_values.get(str(item), item) for item in value
         }

tests/test_fields.py

@@ -1693,6 +1693,23 @@ class TestChoiceField(FieldValues):
             field.run_validation(2)
         assert exc_info.value.detail == ['"2" is not a valid choice.']
 
+    def test_strict_choices(self):
+        """
+        If `strict_choices` then to_representation() should return one of given choices.
+        """
+        field = serializers.ChoiceField(
+            strict_choices=True,
+            choices=[
+                ('poor', 'Poor quality'),
+                ('medium', 'Medium quality'),
+                ('good', 'Good quality'),
+            ]
+        )
+        assert field.to_representation('poor') == 'poor'
+        with pytest.raises(serializers.ValidationError) as exc_info:
+            field.to_representation('amazing')
+        assert exc_info.value.detail == ['"amazing" is not a valid choice.']
+
 
 class TestChoiceFieldWithType(FieldValues):
     """
@@ -1830,6 +1847,24 @@ class TestMultipleChoiceField(FieldValues):
         field.partial = True
         assert field.get_value(QueryDict({})) == rest_framework.fields.empty
 
+    def test_multiple_strict_choices(self):
+        """
+        If `strict_choices` then to_representation() should return a set from given choices.
+        """
+        field = serializers.MultipleChoiceField(
+            strict_choices=True,
+            choices=[
+                ('aircon', 'AirCon'),
+                ('manual', 'Manual drive'),
+                ('diesel', 'Diesel'),
+            ]
+        )
+
+        assert field.to_representation(['aircon', 'manual']) == {'aircon', 'manual'}
+        with pytest.raises(serializers.ValidationError) as exc_info:
+            field.to_representation(['aircon', 'incorrect'])
+        assert exc_info.value.detail == ['"incorrect" is not a valid choice.']
+
 
 class TestEmptyMultipleChoiceField(FieldValues):
     """