fix #1014 - test for no_update by type rather than identity

dash/dash.py

@@ -1357,7 +1357,7 @@ def add_context(*args, **kwargs):
                     has_update = False
                     for i, o in enumerate(output):
                         val = output_value[i]
-                        if val is not no_update:
+                        if not isinstance(val, _NoUpdate):
                             has_update = True
                             o_id, o_prop = o.component_id, o.component_property
                             component_ids[o_id][o_prop] = val
@@ -1367,7 +1367,7 @@ def add_context(*args, **kwargs):
 
                     response = {"response": component_ids, "multi": True}
                 else:
-                    if output_value is no_update:
+                    if isinstance(output_value, _NoUpdate):
                         raise exceptions.PreventUpdate
 
                     response = {

tests/integration/test_integration.py

@@ -2,6 +2,7 @@
 import datetime
 import time
 import pytest
+from copy import copy
 
 from bs4 import BeautifulSoup
 from selenium.webdriver.common.keys import Keys
@@ -527,7 +528,8 @@ def show_clicks(n):
         return [
             no_update if n and n > 4 else n,
             no_update if n and n > 2 else n,
-            no_update,
+            # make a new instance, to mock up caching and restoring no_update
+            copy(no_update),
         ]
 
     dash_duo.start_server(app)