Don't serialize any field if only is empty.

Fixes #772

marshmallow/schema.py

@@ -328,7 +328,7 @@ class Meta:
         """
         pass
 
-    def __init__(self, extra=None, only=(), exclude=(), prefix='', strict=None,
+    def __init__(self, extra=None, only=None, exclude=(), prefix='', strict=None,
                  many=False, context=None, load_only=(), dump_only=(),
                  partial=False):
         # copy declared fields from metaclass
@@ -704,7 +704,7 @@ def _do_load(self, data, many=None, partial=None, postprocess=True):
 
     def _normalize_nested_options(self):
         """Apply then flatten nested schema options"""
-        if self.only:
+        if self.only is not None:
             # Apply the only option to nested fields.
             self.__apply_nested_option('only', self.only)
             # Remove the child field names from the only option.
@@ -737,7 +737,7 @@ def __apply_nested_option(self, option_name, field_names):
 
     def _update_fields(self, obj=None, many=False):
         """Update fields based on the passed in object."""
-        if self.only:
+        if self.only is not None:
             # Return only fields specified in only option
             if self.opts.fields:
                 field_names = self.set_class(self.opts.fields) & self.set_class(self.only)

tests/test_schema.py

@@ -1066,6 +1066,12 @@ class Meta:
     sch = MySchema(only=('baz', ))
     assert sch.dump({'foo': 42}).data == {}
 
+def test_only_empty():
+    class MySchema(Schema):
+        foo = fields.Field()
+
+    sch = MySchema(only=())
+    assert 'foo' not in sch.dump({'foo': 'bar'})
 
 def test_nested_only_and_exclude():
     class Inner(Schema):