simple test demonstrating the issue

rest_framework/fields.py

@@ -1397,7 +1397,7 @@ class ChoiceField(Field):
     html_cutoff_text = _('More than {count} items...')
 
     def __init__(self, choices, **kwargs):
-        self.choices = 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)
 

tests/test_fields.py

@@ -1,6 +1,7 @@
 import datetime
 import os
 import re
+import sys
 import unittest
 import uuid
 from decimal import ROUND_DOWN, ROUND_UP, Decimal
@@ -25,6 +26,13 @@ try:
 except ImportError:
     typings = False
 
+if sys.version_info[0] < 3:
+    # python 2
+    import mock
+else:
+    # python 3
+    from unittest import mock
+
 
 # Tests for helper functions.
 # ---------------------------
@@ -1524,6 +1532,17 @@ class TestChoiceField(FieldValues):
         ]
     )
 
+    @mock.patch('rest_framework.fields.ChoiceField.choices')
+    def test_set_initial_choices(self, choices_mock):
+        field = serializers.ChoiceField(
+            allow_null=True,
+            choices=[
+                1, 2,
+            ]
+        )
+        assert field._choices == [1, 2]
+        assert field.choices == choices_mock
+
     def test_allow_blank(self):
         """
         If `allow_blank=True` then '' is a valid input.