Merge pull request #305 from edx/shr/bug/TNL-2686-etree-set-value-wit…

…h-empty-string-instead-None

return empty string if value is none

xblock/fields.py

@@ -780,11 +780,19 @@ class String(JSONField):
     MUTABLE = False
 
     def from_json(self, value):
-        if value is None or isinstance(value, basestring):
+        if value is None:
+            return ''
+        elif isinstance(value, basestring):
             return value
         else:
             raise TypeError('Value stored in a String must be None or a string, found %s' % type(value))
 
+    def to_json(self, value):
+        if value is None:
+            return ''
+        self._warn_deprecated_outside_JSONField()
+        return value
+
     def from_string(self, value):
         """String gets serialized and deserialized without quote marks."""
         return self.from_json(value)

xblock/test/test_fields.py

@@ -212,7 +212,14 @@ def test_json_equals(self):
         self.assertJSONOrSetEquals('', '')
 
     def test_none(self):
-        self.assertJSONOrSetEquals(None, None)
+        # test for NoneType for to_string and from_string methods
+        test_field = String(enforce_type=True)
+
+        result_to_string = test_field.to_string(None)
+        self.assertEquals(result_to_string, '')
+
+        result_from_string = test_field.from_string(None)
+        self.assertEquals(result_from_string, '')
 
     def test_error(self):
         self.assertJSONOrSetTypeError(['a'])