Alter read_only+default behaviour (#5886)

* Always exclude read_only fields from _writable_fields

* Remove `read_only` from `CreateOnlyDefault` example.
      In this context (without mentioning `save`) now slightly misleading.

docs/api-guide/validators.md

@@ -189,7 +189,6 @@ A default class that can be used to *only set a default argument during create o
 It takes a single argument, which is the default value or callable that should be used during create operations.
 
     created_at = serializers.DateTimeField(
-        read_only=True,
         default=serializers.CreateOnlyDefault(timezone.now)
     )
 

rest_framework/serializers.py

@@ -367,8 +367,7 @@ class Serializer(BaseSerializer):
     @cached_property
     def _writable_fields(self):
         return [
-            field for field in self.fields.values()
-            if (not field.read_only) or (field.default is not empty)
+            field for field in self.fields.values() if not field.read_only
         ]
 
     @cached_property

tests/test_fields.py

@@ -219,10 +219,17 @@ class TestSource:
 class TestReadOnly:
     def setup(self):
         class TestSerializer(serializers.Serializer):
-            read_only = serializers.ReadOnlyField()
+            read_only = serializers.ReadOnlyField(default="789")
             writable = serializers.IntegerField()
         self.Serializer = TestSerializer
 
+    def test_writable_fields(self):
+        """
+        Read-only fields should not be writable, even with default ()
+        """
+        serializer = self.Serializer()
+        assert len(serializer._writable_fields) == 1
+
     def test_validate_read_only(self):
         """
         Read-only serializers.should not be included in validation.

tests/test_serializer.py

@@ -513,7 +513,7 @@ class TestCacheSerializerData:
 class TestDefaultInclusions:
     def setup(self):
         class ExampleSerializer(serializers.Serializer):
-            char = serializers.CharField(read_only=True, default='abc')
+            char = serializers.CharField(default='abc')
             integer = serializers.IntegerField()
         self.Serializer = ExampleSerializer