Merge pull request #680 from plotly/partial-update

Partial updates

Author: alexcjohnson

CHANGELOG.md

@@ -3,6 +3,7 @@
 - Support for "Clientside Callbacks" - an escape hatch to execute your callbacks in JavaScript instead of Python [#672](https://github.com/plotly/dash/pull/672)
 - Added `dev_tools_ui` config flag in `app.run_server` (serialized in `<script id="_dash-config" type="application/json">`)
   to display or hide the forthcoming Dev Tools UI in Dash's front-end (dash-renderer). [#676](https://github.com/plotly/dash/pull/676)
+- Partial updates: leave some multi-output updates unchanged while updating others [#680](https://github.com/plotly/dash/pull/680)
 
 ## [0.40.0] - 2019-03-25
 ### Changed

dash/__init__.py

@@ -1,4 +1,4 @@
-from .dash import Dash  # noqa: F401
+from .dash import Dash, no_update  # noqa: F401
 from . import dependencies  # noqa: F401
 from . import development  # noqa: F401
 from . import exceptions  # noqa: F401

dash/dash.py

@@ -74,6 +74,15 @@
 _re_renderer_scripts_id = re.compile(r'id="_dash-renderer')
 
 
+class _NoUpdate(object):
+    # pylint: disable=too-few-public-methods
+    pass
+
+
+# Singleton signal to not update an output, alternative to PreventUpdate
+no_update = _NoUpdate()
+
+
 # pylint: disable=too-many-instance-attributes
 # pylint: disable=too-many-arguments, too-many-locals
 class Dash(object):
@@ -1053,15 +1062,25 @@ def add_context(*args, **kwargs):
                         )
 
                     component_ids = collections.defaultdict(dict)
+                    has_update = False
                     for i, o in enumerate(output):
-                        component_ids[o.component_id][o.component_property] =\
-                            output_value[i]
+                        val = output_value[i]
+                        if val is not no_update:
+                            has_update = True
+                            o_id, o_prop = o.component_id, o.component_property
+                            component_ids[o_id][o_prop] = val
+
+                    if not has_update:
+                        raise exceptions.PreventUpdate
 
                     response = {
                         'response': component_ids,
                         'multi': True
                     }
                 else:
+                    if output_value is no_update:
+                        raise exceptions.PreventUpdate
+
                     response = {
                         'response': {
                             'props': {

tests/IntegrationTests.py

@@ -21,15 +21,23 @@ def percy_snapshot(cls, name=''):
             name=snapshot_name
         )
 
-    def wait_for_element_by_css_selector(self, selector):
-        return WebDriverWait(self.driver, TIMEOUT).until(
-            EC.presence_of_element_located((By.CSS_SELECTOR, selector))
+    def wait_for_element_by_css_selector(self, selector, timeout=TIMEOUT):
+        return WebDriverWait(self.driver, timeout).until(
+            EC.presence_of_element_located((By.CSS_SELECTOR, selector)),
+            'Could not find element with selector "{}"'.format(selector)
         )
 
-    def wait_for_text_to_equal(self, selector, assertion_text):
-        return WebDriverWait(self.driver, TIMEOUT).until(
-            EC.text_to_be_present_in_element((By.CSS_SELECTOR, selector),
-                                             assertion_text)
+    def wait_for_text_to_equal(self, selector, assertion_text, timeout=TIMEOUT):
+        el = self.wait_for_element_by_css_selector(selector)
+        WebDriverWait(self.driver, timeout).until(
+            lambda *args: (
+                (str(el.text) == assertion_text) or
+                (str(el.get_attribute('value')) == assertion_text)
+            ),
+            "Element '{}' text was supposed to equal '{}' but it didn't".format(
+                selector,
+                assertion_text
+            )
         )
 
     @classmethod