added TreeExpander example

docs/experimental/loaders.rst

@@ -404,6 +404,17 @@ one, as the other is just an identity expander.
     Loader arguments passed to on each expander call. This arguments can be
     overriden with specific noader arguments passed on each expander call.
 
+    Example::
+
+        >>> def filter_world(x):
+        ...     return None if x == 'world' else x
+        ...
+        >>> from scrapy.newitem.loader.expanders import TreeExpander
+        >>> expander = TreeExpander(filter_world, str.upper)
+        >>> expander(['hello', 'world', 'this', 'is', 'scrapy'])
+        ['HELLO, 'THIS', 'IS', 'SCRAPY']
+
+
 IdentityExpander
 ----------------
 

scrapy/newitem/loader/expanders.py

@@ -26,7 +26,7 @@ class TreeExpander(object):
     def wrap_with_args(self, f):
         return lambda x, y: f(x, loader_args=y)
 
-    def __call__(self, value, loader_args):
+    def __call__(self, value, loader_args=None):
         values = arg_to_iter(value)
         largs = self.default_loader_args
         if loader_args:

scrapy/tests/test_itemloader.py

@@ -70,6 +70,14 @@ class LoaderTest(unittest.TestCase):
         self.assertEqual(il.get_expanded_value('name'), [u'Pepe'])
         self.assertEqual(il.get_reduced_value('name'), u'Pepe')
 
+    def test_tree_expander_filter(self):
+        def filter_world(x):
+            return None if x == 'world' else x
+
+        expander = TreeExpander(filter_world, str.upper)
+        self.assertEqual(expander(['hello', 'world', 'this', 'is', 'scrapy']),
+                         ['HELLO', 'THIS', 'IS', 'SCRAPY'])
+
     def test_tree_expander_multiple_functions(self):
         class TestLoader(NameLoader):
             name_exp = TreeExpander(lambda v: v.title(), lambda v: v[:-1])