Add handling of PreventUpdate exceptions (#208)

* Handle the raising of dash.exceptions.PreventUpdate

* Missing demo file and minor formatting tweaks

* Add in missing copyright notice

demo/demo/plotly_apps.py

@@ -41,6 +41,8 @@
 
 import dpd_components as dpd
 
+from dash.exceptions import PreventUpdate
+
 from django_plotly_dash import DjangoDash
 from django_plotly_dash.consumers import send_to_pipe_channel
 
@@ -336,8 +338,8 @@ def callback_show_timeseries(internal_state_string, state_uid, **kwargs):
      dash.dependencies.Output('output-two', 'children'),
      dash.dependencies.Output('output-three', 'children')
      ],
-    [dash.dependencies.Input('button','n_clicks'),
-     dash.dependencies.Input('dropdown-color','value'),
+    [dash.dependencies.Input('button', 'n_clicks'),
+     dash.dependencies.Input('dropdown-color', 'value'),
      ])
 def multiple_callbacks_one(button_clicks, color_choice):
     return ("Output 1: %s %s" % (button_clicks, color_choice),
@@ -365,12 +367,13 @@ def multiple_callbacks_one(button_clicks, color_choice):
      dash.dependencies.Output('output-two', 'children'),
      dash.dependencies.Output('output-three', 'children')
      ],
-    [dash.dependencies.Input('button','n_clicks'),
-     dash.dependencies.Input('dropdown-color','value'),
+    [dash.dependencies.Input('button', 'n_clicks'),
+     dash.dependencies.Input('dropdown-color', 'value'),
      ])
 def multiple_callbacks_two(button_clicks, color_choice, **kwargs):
+    if color_choice == 'green':
+        raise PreventUpdate
     return ["Output 1: %s %s" % (button_clicks, color_choice),
             "Output 2: %s %s" % (button_clicks, color_choice),
             "Output 3: %s %s [%s]" % (button_clicks, color_choice, kwargs)
             ]
-

demo/demo/templates/demo_ten.html

@@ -9,6 +9,11 @@ <h1>Multiple Callback Values</h1>
   Both standard and extended callbacks can return multiple responses. This example instantiates
   two example applications that use multiple return values for both types of callback.
 </p>
+<p>
+  Note that the second application will not update if the color choice is set to Green. This is
+  intentional; the callback raises a <b>dash.exceptions.PreventUpdate</b> exception when this
+  color choice is in effect.
+</p>
 <div class="card bg-light border-dark">
   <div class="card-body">
     <p><span>{</span>% load plotly_dash %}</p>

demo/demo/tests/test_dpd_demo.py

@@ -1,5 +1,26 @@
 '''
 Test django_plotly_dash functionality by use of demo pages
+
+Copyright (c) 2018 Gibbs Consulting and others - see CONTRIBUTIONS.md
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+
 '''
 
 import pytest

django_plotly_dash/views.py

@@ -29,6 +29,8 @@
 from django.http import HttpResponse, HttpResponseRedirect
 from django.shortcuts import redirect
 
+from dash.exceptions import PreventUpdate
+
 try:
     from dash.fingerprint import check_fingerprint
 except:
@@ -68,6 +70,12 @@ def layout(request, ident, stateless=False, cache_id=None, **kwargs):
                         content_type=mimetype)
 
 def update(request, ident, stateless=False, **kwargs):
+    try:
+        return _update(request, ident, stateless, **kwargs)
+    except PreventUpdate:
+        return HttpResponse(status=204)
+
+def _update(request, ident, stateless=False, **kwargs):
     'Generate update json response'
     dash_app, app = DashApp.locate_item(ident, stateless)