def deserialize_tags(self, tags):
         if isinstance(tags, basestring):
             return tags.split(',') if tags else []
-        return tags
+        return tags or []
 
     def serialize_depends(self, cur_dependencies):
         # Return the list of uuids
     read_only_fields = []
 
     def _load_data(self, data):
-        self._data = data
+        self._data = dict((key, self._deserialize(key, value))
+                          for key, value in data.items())
         # We need to use a copy for original data, so that changes
-        # are not propagated. Shallow copy is alright, since data dict uses only
-        # primitive data types
-        self._original_data = data.copy()
-
-    def _update_data(self, data, update_original=False):
-        """
-        Low level update of the internal _data dict. Data which are coming as
-        updates should already be serialized. If update_original is True, the
-        original_data dict is updated as well.
-        """
-
-        self._data.update(data)
-
-        if update_original:
-            self._original_data.update(data)
+        # are not propagated.
+        self._original_data = copy.deepcopy(self._data)
 
     def __getitem__(self, key):
         # This is a workaround to make TaskResource non-iterable
         except ValueError:
             pass
 
-        return self._deserialize(key, self._data.get(key))
+        if key not in self._data:
+            self._data[key] = self._deserialize(key, None)
+
+        return self._data.get(key)
 
     def __setitem__(self, key, value):
         if key in self.read_only_fields:
             raise RuntimeError('Field \'%s\' is read-only' % key)
-        self._data[key] = self._serialize(key, value)
+        self._data[key] = value
 
     def __str__(self):
         s = six.text_type(self.__unicode__())
             if self._data.get(key) != self._original_data.get(key):
                 yield key
 
+    @property
+    def _is_modified(self):
+        return bool(list(self._modified_fields))
+
     @property
     def completed(self):
         return self['status'] == six.text_type('completed')
         # to keep a list of all depedencies in the _data dictionary,
         # not just currently added/removed ones
 
-        old_dependencies_raw = self._original_data.get('depends','')
-        old_dependencies = self.deserialize_depends(old_dependencies_raw)
+        old_dependencies = self._original_data.get('depends', set())
 
         added = self['depends'] - old_dependencies
         removed = old_dependencies - self['depends']
             raise Task.NotSaved("Task needs to be saved before it can be deleted")
 
         # Refresh the status, and raise exception if the task is deleted
-        self.refresh(only_fields=['status'])
+        self.refresh()
 
         if self.deleted:
             raise Task.DeletedTask("Task was already deleted")
         self.warrior.execute_command([self['uuid'], 'delete'])
 
         # Refresh the status again, so that we have updated info stored
-        self.refresh(only_fields=['status'])
+        self.refresh()
 
 
     def done(self):
             raise Task.NotSaved("Task needs to be saved before it can be completed")
 
         # Refresh, and raise exception if task is already completed/deleted
-        self.refresh(only_fields=['status'])
+        self.refresh()
 
         if self.completed:
             raise Task.CompletedTask("Cannot complete a completed task")
         self.warrior.execute_command([self['uuid'], 'done'])
 
         # Refresh the status again, so that we have updated info stored
-        self.refresh(only_fields=['status'])
+        self.refresh()
 
     def save(self):
+        if self.saved and not self._is_modified:
+            return
+
         args = [self['uuid'], 'modify'] if self.saved else ['add']
         args.extend(self._get_modified_fields_as_args())
         output = self.warrior.execute_command(args)
 
         args = [self['uuid'], 'annotate', annotation]
         self.warrior.execute_command(args)
-        self.refresh(only_fields=['annotations'])
+        self.refresh()
 
     def remove_annotation(self, annotation):
         if not self.saved:
             annotation = annotation['description']
         args = [self['uuid'], 'denotate', annotation]
         self.warrior.execute_command(args)
-        self.refresh(only_fields=['annotations'])
+        self.refresh()
 
     def _get_modified_fields_as_args(self):
         args = []
 
         def add_field(field):
             # Add the output of format_field method to args list (defaults to
-            # field:'value')
-            format_default = lambda k: "{0}:{1}".format(k,
-                                           "'{0}'".format(self._data[k])
-                                           if self._data[k] is not None
-                                           else '')
+            # field:value)
+            serialized_value = self._serialize(field, self._data[field]) or ''
+            format_default = lambda: "{0}:{1}".format(
+                field,
+                "'{0}'".format(serialized_value) if serialized_value else ''
+            )
             format_func = getattr(self, 'format_{0}'.format(field),
-                                  lambda: format_default(field))
+                                  format_default)
             args.append(format_func())
 
         # If we're modifying saved task, simply pass on all modified fields
 
         return args
 
-    def refresh(self, only_fields=[]):
+    def refresh(self):
         # Raise error when trying to refresh a task that has not been saved
         if not self.saved:
             raise Task.NotSaved("Task needs to be saved to be refreshed")
         # with using UUID only.
         args = [self['uuid'] or self['id'], 'export']
         new_data = json.loads(self.warrior.execute_command(args)[0])
-        if only_fields:
-            to_update = dict(
-                [(k, new_data.get(k)) for k in only_fields])
-            self._update_data(to_update, update_original=True)
-        else:
-            self._load_data(new_data)
+        self._load_data(new_data)
 
 
 class TaskFilter(SerializingObject):
 
 
         # We only save the parent task, dependency task is unsaved
         t.save()
+        t['depends'] = set([dependency])
 
-        self.assertRaises(Task.NotSaved,
-                          t.__setitem__, 'depends', set([dependency]))
+        self.assertRaises(Task.NotSaved, t.save)
 
     def test_set_simple_dependency_set(self):
         # Adds only one dependency to task with no dependencies
         t['depends'] = set([dependency1, dependency2])
         t.save()
 
-        t['depends'] = t['depends'] - set([dependency2])
+        t['depends'].remove(dependency2)
         t.save()
 
         self.assertEqual(t['depends'], set([dependency1]))
         t['depends'] = set([dependency1])
         t.save()
 
-        t['depends'] = t['depends'] | set([dependency2])
+        t['depends'].add(dependency2)
         t.save()
 
         self.assertEqual(t['depends'], set([dependency1, dependency2]))
 
+    def test_add_to_empty_dependency_set(self):
+        # Adds dependency to task with one dependencies
+        t = Task(self.tw, description="test task")
+        dependency = Task(self.tw, description="needs to be done first")
+
+        dependency.save()
+
+        t['depends'].add(dependency)
+        t.save()
+
+        self.assertEqual(t['depends'], set([dependency]))
+
     def test_simple_dependency_set_save_repeatedly(self):
         # Adds only one dependency to task with no dependencies
         t = Task(self.tw, description="test task")
             self.assertRaises(RuntimeError,
                               lambda: t.__setitem__(readonly_key, 'value'))
 
+    def test_saving_unmodified_task(self):
+        t = Task(self.tw, description="test task")
+        t.save()
+        t.save()
+
+    def test_adding_tag_by_appending(self):
+        t = Task(self.tw, description="test task", tags=['test1'])
+        t.save()
+        t['tags'].append('test2')
+        t.save()
+        self.assertEqual(t['tags'], ['test1', 'test2'])
+
+    def test_adding_tag_by_appending_empty(self):
+        t = Task(self.tw, description="test task")
+        t.save()
+        t['tags'].append('test')
+        t.save()
+        self.assertEqual(t['tags'], ['test'])
+
 
 class AnnotationTest(TasklibTest):