Add saves_null_values attribute to Field

import_export/fields.py

@@ -27,18 +27,21 @@ class Field(object):
     :param default: This value will be returned by
         :meth:`~import_export.fields.Field.clean` if this field's widget did
         not return an adequate value.
+
+    :param saves_null_values: Controls whether null values are saved on the object
     """
     empty_values = [None, '']
 
     def __init__(self, attribute=None, column_name=None, widget=None,
-                 default=NOT_PROVIDED, readonly=False):
+                 default=NOT_PROVIDED, readonly=False, saves_null_values=True):
         self.attribute = attribute
         self.default = default
         self.column_name = column_name
         if not widget:
             widget = widgets.Widget()
         self.widget = widget
         self.readonly = readonly
+        self.saves_null_values = saves_null_values
 
     def __repr__(self):
         """
@@ -109,7 +112,8 @@ def save(self, obj, data):
             attrs = self.attribute.split('__')
             for attr in attrs[:-1]:
                 obj = getattr(obj, attr, None)
-            setattr(obj, attrs[-1], self.clean(data))
+            if self.clean(data) != None or self.saves_null_values:
+                setattr(obj, attrs[-1], self.clean(data))
 
     def export(self, obj):
         """

tests/core/tests/fields_tests.py

@@ -64,3 +64,14 @@ def test_default_falsy_values(self):
     def test_default_falsy_values_without_default(self):
         field = fields.Field(column_name='name')
         self.assertEqual(field.clean({'name': 0}), 0)
+
+    def test_saves_null_values(self):
+        field = fields.Field(column_name='name', attribute='name', saves_null_values=False)
+        row = {
+            'name': None,
+        }
+        field.save(self.obj, row)
+        self.assertEqual(self.obj.name, 'Foo')
+
+        self.field.save(self.obj, row)
+        self.assertIsNone(self.obj.name)