Allow `ChoiceField.choices` to be set dynamically (#5426)

## Description

The `choices` field for the `ChoiceField` class should be able to be edited after `ChoiceField.__init__` is called.

```
field = ChoiceField(choices=[1,2])
field.choices = [1]  # Should no longer allow `2` as a choice
```

Currently, you must update `choices`, `grouped_choices`, and `choice_strings_to_values` to achieve this. This P/R keeps `grouped_choices` and `choice_strings_to_values` in sync whenever the `choices` are edited.

rest_framework/fields.py

@@ -1337,18 +1337,10 @@ class ChoiceField(Field):
     html_cutoff_text = _('More than {count} items...')
 
     def __init__(self, choices, **kwargs):
-        self.grouped_choices = to_choices_dict(choices)
-        self.choices = flatten_choices_dict(self.grouped_choices)
+        self.choices = choices
         self.html_cutoff = kwargs.pop('html_cutoff', self.html_cutoff)
         self.html_cutoff_text = kwargs.pop('html_cutoff_text', self.html_cutoff_text)
 
-        # Map the string representation of choices to the underlying value.
-        # Allows us to deal with eg. integer choices while supporting either
-        # integer or string input, but still get the correct datatype out.
-        self.choice_strings_to_values = {
-            six.text_type(key): key for key in self.choices.keys()
-        }
-
         self.allow_blank = kwargs.pop('allow_blank', False)
 
         super(ChoiceField, self).__init__(**kwargs)
@@ -1377,6 +1369,22 @@ class ChoiceField(Field):
             cutoff_text=self.html_cutoff_text
         )
 
+    def _get_choices(self):
+        return self._choices
+
+    def _set_choices(self, choices):
+        self.grouped_choices = to_choices_dict(choices)
+        self._choices = flatten_choices_dict(self.grouped_choices)
+
+        # Map the string representation of choices to the underlying value.
+        # Allows us to deal with eg. integer choices while supporting either
+        # integer or string input, but still get the correct datatype out.
+        self.choice_strings_to_values = {
+            six.text_type(key): key for key in self.choices.keys()
+        }
+
+    choices = property(_get_choices, _set_choices)
+
 
 class MultipleChoiceField(ChoiceField):
     default_error_messages = {

tests/test_fields.py

@@ -1425,6 +1425,19 @@ class TestChoiceField(FieldValues):
 
         assert items[9].value == 'boolean'
 
+    def test_edit_choices(self):
+        field = serializers.ChoiceField(
+            allow_null=True,
+            choices=[
+                1, 2,
+            ]
+        )
+        field.choices = [1]
+        assert field.run_validation(1) is 1
+        with pytest.raises(serializers.ValidationError) as exc_info:
+            field.run_validation(2)
+        assert exc_info.value.detail == ['"2" is not a valid choice.']
+
 
 class TestChoiceFieldWithType(FieldValues):
     """