Merge pull request #2425 from plotly/logging-fix

Move logger to dash namespace, add stream handler once.

CHANGELOG.md

@@ -8,11 +8,17 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 
 - [#2417](https://github.com/plotly/dash/pull/2417) Add wait_timeout property to customize the behavior of the default wait timeout used for by wait_for_page, fix [#1595](https://github.com/plotly/dash/issues/1595)
 - [#2417](https://github.com/plotly/dash/pull/2417) Add the element target text for wait_for_text* error message, fix [#945](https://github.com/plotly/dash/issues/945)
+- [#2425](https://github.com/plotly/dash/pull/2425) Add `add_log_handler=True` to Dash init, if you don't want a log stream handler at all.
 
 ## Fixed
 
 - [#2417](https://github.com/plotly/dash/pull/2417) Disable the pytest plugin if `dash[testing]` not installed, fix [#946](https://github.com/plotly/dash/issues/946).
 - [#2417](https://github.com/plotly/dash/pull/2417) Do not swallow the original error to get the webdriver, easier to know what is wrong after updating the browser but the driver.
+- [#2425](https://github.com/plotly/dash/pull/2425) Fix multiple log handler added unconditionally to the logger, resulting in duplicate log message.
+
+## Changed
+
+- [#2425](https://github.com/plotly/dash/pull/2425) Moved the logger namespace to `dash.dash`, as library logger it should be on that namespace instead of the user app.
 
 ## [UNRELEASED]
 

dash/dash.py

@@ -329,6 +329,9 @@ class Dash:
     :param background_callback_manager: Background callback manager instance
         to support the ``@callback(..., background=True)`` decorator.
         One of ``DiskcacheManager`` or ``CeleryManager`` currently supported.
+
+    :param add_log_handler: Automatically add a StreamHandler to the app logger
+        if not added previously.
     """
 
     def __init__(  # pylint: disable=too-many-statements
@@ -361,6 +364,7 @@ def __init__(  # pylint: disable=too-many-statements
         update_title="Updating...",
         long_callback_manager=None,
         background_callback_manager=None,
+        add_log_handler=True,
         **obsolete,
     ):
         _validate.check_obsolete(obsolete)
@@ -481,8 +485,10 @@ def __init__(  # pylint: disable=too-many-statements
         self._long_callback_count = 0
         self._background_manager = background_callback_manager or long_callback_manager
 
-        self.logger = logging.getLogger(name)
-        self.logger.addHandler(logging.StreamHandler(stream=sys.stdout))
+        self.logger = logging.getLogger(__name__)
+
+        if not self.logger.handlers and add_log_handler:
+            self.logger.addHandler(logging.StreamHandler(stream=sys.stdout))
 
         if isinstance(plugins, patch_collections_abc("Iterable")):
             for plugin in plugins: