add the default parameter to get_output_value

itemloaders/__init__.py

@@ -266,7 +266,7 @@ def load_item(self):
 
         return adapter.item
 
-    def get_output_value(self, field_name):
+    def get_output_value(self, field_name, default=None):
         """
         Return the collected values parsed using the output processor, for the
         given field. This method doesn't populate or modify the item at all.
@@ -275,7 +275,10 @@ def get_output_value(self, field_name):
         proc = wrap_loader_context(proc, self.context)
         value = self._values.get(field_name, [])
         try:
-            return proc(value)
+            result = proc(value)
+            if not result and default is not None:
+                return default
+            return result
         except Exception as e:
             raise ValueError("Error with output processor: field=%r value=%r error='%s: %s'" %
                              (field_name, value, type(e).__name__, str(e)))

main.py

@@ -0,0 +1,6 @@
+from itemloaders import ItemLoader
+from itemloaders.processors import MapCompose, TakeFirst
+
+
+class CustomItemLoader(ItemLoader):
+    name_out = TakeFirst()

tests/test_base_loader.py

@@ -68,7 +68,7 @@ class MyLoader(ItemLoader):
             url_out = TakeFirst()
 
             def img_url_out(self, values):
-                return (self.get_output_value('url') or '') + values[0]
+                return self.get_output_value('url', '') + values[0]
 
         il = MyLoader(item={})
         il.add_value('url', 'http://example.com/')

tests/test_loader_initialization.py

@@ -1,6 +1,7 @@
 import unittest
 
 from itemloaders import ItemLoader
+from itemloaders.processors import TakeFirst
 
 
 class InitializationTestMixin:
@@ -77,6 +78,53 @@ def test_get_output_value_list(self):
         self.assertIsInstance(loaded_item, self.item_class)
         self.assertEqual(loaded_item, dict({'name': ['foo', 'bar']}))
 
+    def test_get_output_value_default_singlevalue(self):
+        """
+        The default value should be used only when the returned value is
+        empty (None, '', etc.) and there is a default value defined
+        """
+        input_item = self.item_class()
+        il = ItemLoader(item=input_item)
+        il.default_output_processor = TakeFirst()  # make "name" field single
+
+        self.assertEqual(il.get_output_value('name'), None)
+        self.assertEqual(il.get_output_value('name', ''), '')
+        self.assertEqual(il.get_output_value('name', []), [])
+        self.assertEqual(il.get_output_value('name', 'foo'), 'foo')
+
+        il.add_value('name', '')
+        self.assertEqual(il.get_output_value('name'), None)
+        self.assertEqual(il.get_output_value('name', ''), '')
+        self.assertEqual(il.get_output_value('name', 'foo'), 'foo')
+        self.assertEqual(il.load_item(), {})
+
+        input_item2 = self.item_class()
+        il2 = ItemLoader(item=input_item2)
+        il2.default_output_processor = TakeFirst()
+        il2.add_value('name', 'foo')
+        self.assertEqual(il2.get_output_value('name'), 'foo')
+        self.assertEqual(il2.get_output_value('name', 'bar'), 'foo')
+        self.assertEqual(il2.load_item(), dict({'name': 'foo'}))
+
+    def test_get_output_value_default_list(self):
+        """
+        The default value should be used only when the returned value is
+        empty ([], etc.) and there is a default value defined
+        """
+        input_item = self.item_class()
+        il = ItemLoader(item=input_item)
+        il.add_value('name', [])
+        self.assertEqual(il.get_output_value('name'), [])
+        self.assertEqual(il.get_output_value('name', 'foo'), 'foo')
+        self.assertEqual(il.load_item(), {})
+
+        input_item2 = self.item_class()
+        il2 = ItemLoader(item=input_item2)
+        il2.add_value('name', ['foo', 'bar'])
+        self.assertEqual(il2.get_output_value('name'), ['foo', 'bar'])
+        self.assertEqual(il2.get_output_value('name', ['spam']), ['foo', 'bar'])
+        self.assertEqual(il2.load_item(), dict({'name': ['foo', 'bar']}))
+
     def test_values_single(self):
         """Values from initial item must be added to loader._values"""
         input_item = self.item_class(name='foo')