Failing Test Case for #5345

Added a failing test case for Github issue #5345 to test whether the
.initial_data passed to a serializer's .validate() method is a
dict (pass) or a list (fail) of the appropriate length.

Signed-off-by: Rod Manning <rod.manning@taslytic.com>

tests/test_initial_data.py

@@ -0,0 +1,43 @@
+from rest_framework import serializers
+
+
+class TestInitialData:
+    """
+    Reference github issue #5345.
+
+    Tests that the .initial_data the ListSerializer is updating the
+    initial_data of it's child serializer as it is iterating.
+
+    This test is done by checking that:
+
+    1. The length of the .initial_data is equal to the number of items included
+       in the test case's input_data, and
+
+    2. Test that the .initial_data passed to the .validate() function is a
+       dict.
+
+    (Note that the assert statements are included in the .validate() method as
+    this is where we want to look at the data to verify it's behaviour.)
+
+    """
+
+    def setup(self):
+        class TestSerializer(serializers.Serializer):
+            number = serializers.IntegerField()
+
+            def validate(self, attrs):
+                assert len(self.initial_data) == 1
+                assert isinstance(self.initial_data, dict)
+                return attrs
+
+        self.Serializer = TestSerializer
+
+    def test_initial_data(self):
+
+        input_data = {"number": 1}
+        serializer = self.Serializer(data=input_data)
+        serializer.is_valid()
+
+        input_data = [{"number": 1}, {"number": 2}]
+        list_serializer = self.Serializer(data=input_data, many=True)
+        list_serializer.is_valid()