Merge branch 'missing-drop' of github.com:jayd3e/colander into jayd3e…

…-missing-drop

colander/__init__.py

@@ -40,6 +40,15 @@ def __reduce__(self):
 
 null = _null()
 
+class _drop(object):
+    """
+    Represents a value that should be dropped if it is missing during
+    deserialization.
+    """
+    pass
+
+drop = _drop()
+
 def interpolate(msgs):
     for s in msgs:
         if hasattr(s, 'interpolate'):
@@ -532,11 +541,14 @@ def _impl(self, node, value, callback):
             name = subnode.name
             subval = value.pop(name, null)
             try:
-                result[name] = callback(subnode, subval)
+                sub_result = callback(subnode, subval)
             except Invalid as e:
                 if error is None:
                     error = Invalid(node)
                 error.add(e, num)
+            else:
+                if sub_result is not drop:
+                    result[name] = sub_result
 
         if self.unknown == 'raise':
             if value:
@@ -1641,7 +1653,9 @@ class _SchemaNode(object):
       not provided, the missing value of this node will be the special marker
       value :attr:`colander.required`, indicating that it is considered
       'required'.  When ``missing`` is :attr:`colander.required`, the
-      ``required`` computed attribute will be ``True``.
+      ``required`` computed attribute will be ``True``.  When ``missing`` is
+      :attr:`colander.drop`, the node is dropped from the schema if it isn't
+      set during serialization/deserialization.
 
     - ``preparer``: Optional preparer for this node.  It should be
       an object that implements the

colander/tests/test_colander.py

@@ -2897,6 +2897,16 @@ class MySchema(colander.Schema):
         self.assertEqual(node.children[2].title, '')
         self.assertEqual(node.children[3].title, 'thing2')
 
+    def test_deserialize_drop(self):
+        import colander
+        class MySchema(colander.Schema):
+            a = colander.SchemaNode(colander.String())
+            b = colander.SchemaNode(colander.String(), missing=colander.drop)
+        node = MySchema()
+        expected = {'a': 'test'}
+        result = node.deserialize(expected)
+        self.assertEqual(result, expected)
+
 class TestSequenceSchema(unittest.TestCase):
     def test_succeed(self):
         import colander

docs/api.rst

@@ -139,3 +139,7 @@ Schema-Related
   .. attribute:: required
 
      Represents a required value in colander-related operations.
+
+  .. attribute:: drop
+     Represents a value that will be dropped from the schema if it is missing.  Passed
+     ass a value to the `missing` keyword argument.