Skip null attributes post serialization for maps (pynamodb#455)

Jonathan Como · harleyk · commit ae03f5571249 · 2018-03-14T13:54:30.000-07:00
* Skip null attributes post serialization for maps
diff --git a/pynamodb/attributes.py b/pynamodb/attributes.py
@@ -735,9 +735,7 @@ def serialize(self, values):
         rval = {}
         for k in values:
             v = values[k]
-            # Continue to serialize NULL values in "raw" map attributes for backwards compatibility.
-            # This special case behavior for "raw" attribtues should be removed in the future.
-            if not self.is_raw() and v is None:
+            if self._should_skip(v):
                 continue
             attr_class = self._get_serialize_class(k, v)
             if attr_class is None:
@@ -751,7 +749,12 @@ def serialize(self, values):
             attr = self._get_attributes().get(k)
             attr_name = attr.attr_name if attr else k
 
-            rval[attr_name] = {attr_key: attr_class.serialize(v)}
+            serialized = attr_class.serialize(v)
+            if self._should_skip(serialized):
+                # Check after we serialize in case the serialized value is null
+                continue
+
+            rval[attr_name] = {attr_key: serialized}
 
         return rval
 
@@ -788,6 +791,11 @@ def as_dict(self):
             result[key] = value.as_dict() if isinstance(value, MapAttribute) else value
         return result
 
+    def _should_skip(self, value):
+        # Continue to serialize NULL values in "raw" map attributes for backwards compatibility.
+        # This special case behavior for "raw" attribtues should be removed in the future.
+        return not self.is_raw() and value is None
+
     @classmethod
     def _get_serialize_class(cls, key, value):
         if not cls.is_raw():
diff --git a/pynamodb/tests/test_attributes.py b/pynamodb/tests/test_attributes.py
@@ -51,6 +51,7 @@ class CustomAttrMap(MapAttribute):
 
 class DefaultsMap(MapAttribute):
     map_field = MapAttribute(default={})
+    string_field = UnicodeAttribute(null=True)
 
 
 class TestAttributeDescriptor:
@@ -574,6 +575,14 @@ def test_null_attribute_subclassed_map(self):
         serialized = attr.serialize(null_attribute)
         assert serialized == {}
 
+    def test_null_attribute_map_after_serialization(self):
+        null_attribute = {
+            'string_field': '',
+        }
+        attr = DefaultsMap()
+        serialized = attr.serialize(null_attribute)
+        assert serialized == {}
+
     def test_map_of_map(self):
         attribute = {
             'name': 'Justin',
