From 134f5fa4bc783481d394f9c35d9e9e5880c30103 Mon Sep 17 00:00:00 2001
From: paolopaolopaolo <dpaolomercado@gmail.com>
Date: Tue, 22 Sep 2015 11:40:19 -0700
Subject: [PATCH 1/2] Fixes #3265 (now with Test Case)

- Added test_data_access_before_save_raises_error test
---
 tests/test_serializer.py | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/tests/test_serializer.py b/tests/test_serializer.py
index c18cbb584..e99efe3d2 100644
--- a/tests/test_serializer.py
+++ b/tests/test_serializer.py
@@ -51,6 +51,15 @@ class TestSerializer:
         with pytest.raises(AttributeError):
             serializer.data
 
+    def test_data_access_before_save_raises_error(self):
+        def create(validated_data):
+            return validated_data
+        serializer = self.Serializer(data={'char': 'abc', 'integer': 123})
+        serializer.create = create
+        assert serializer.is_valid()
+        assert serializer.data == {'char': 'abc', 'integer': 123}
+        with pytest.raises(AssertionError):
+            serializer.save()
 
 class TestValidateMethod:
     def test_non_field_error_validate_method(self):

From 7640bfea9ed80bd2cb892efebd75a797681f5297 Mon Sep 17 00:00:00 2001
From: paolopaolopaolo <dpaolomercado@gmail.com>
Date: Tue, 22 Sep 2015 11:49:51 -0700
Subject: [PATCH 2/2] Add `assert` statement to `.save()` method in Serializer:

- Asserts that `_data` does not exist when calling `.save()`
---
 rest_framework/serializers.py | 6 ++++++
 tests/test_serializer.py      | 1 +
 2 files changed, 7 insertions(+)

diff --git a/rest_framework/serializers.py b/rest_framework/serializers.py
index 037939e31..a3c26e1ee 100644
--- a/rest_framework/serializers.py
+++ b/rest_framework/serializers.py
@@ -166,6 +166,12 @@ class BaseSerializer(Field):
             "For example: 'serializer.save(owner=request.user)'.'"
         )
 
+        assert not hasattr(self, '_data'), (
+            "You cannot call `.save()` after accessing `serializer.data`."
+            "If you need to access data before committing to the database then "
+            "inspect 'serializer.validated_data' instead. "
+        )
+
         validated_data = dict(
             list(self.validated_data.items()) +
             list(kwargs.items())
diff --git a/tests/test_serializer.py b/tests/test_serializer.py
index e99efe3d2..741c6ab17 100644
--- a/tests/test_serializer.py
+++ b/tests/test_serializer.py
@@ -61,6 +61,7 @@ class TestSerializer:
         with pytest.raises(AssertionError):
             serializer.save()
 
+
 class TestValidateMethod:
     def test_non_field_error_validate_method(self):
         class ExampleSerializer(serializers.Serializer):