From 9fcffd78ce08d164a018105bb9417b38faaec629 Mon Sep 17 00:00:00 2001
From: Ran Benita <ran234@gmail.com>
Date: Thu, 9 May 2019 09:33:17 +0300
Subject: [PATCH] Don't cache _readable_fields and _writable_fields

It might be useful for a serializer with many many fields which uses
read_only and write_only on a large percentage of the fields. But the
memory usage and statefulness it adds are not worth it for the common
case.
---
 rest_framework/serializers.py | 17 ++++++++---------
 tests/test_fields.py          |  2 +-
 2 files changed, 9 insertions(+), 10 deletions(-)

diff --git a/rest_framework/serializers.py b/rest_framework/serializers.py
index 7b7959db9..b23389c56 100644
--- a/rest_framework/serializers.py
+++ b/rest_framework/serializers.py
@@ -361,18 +361,17 @@ class Serializer(BaseSerializer, metaclass=SerializerMetaclass):
             fields[key] = value
         return fields
 
-    @cached_property
+    @property
     def _writable_fields(self):
-        return [
-            field for field in self.fields.values() if not field.read_only
-        ]
+        for field in self.fields.values():
+            if not field.read_only:
+                yield field
 
-    @cached_property
+    @property
     def _readable_fields(self):
-        return [
-            field for field in self.fields.values()
-            if not field.write_only
-        ]
+        for field in self.fields.values():
+            if not field.write_only:
+                yield field
 
     def get_fields(self):
         """
diff --git a/tests/test_fields.py b/tests/test_fields.py
index e7f16c178..468b33e57 100644
--- a/tests/test_fields.py
+++ b/tests/test_fields.py
@@ -219,7 +219,7 @@ class TestReadOnly:
         Read-only fields should not be writable, even with default ()
         """
         serializer = self.Serializer()
-        assert len(serializer._writable_fields) == 1
+        assert len(list(serializer._writable_fields)) == 1
 
     def test_validate_read_only(self):
         """