Add callback on_error handler

.pylintrc

@@ -414,7 +414,7 @@ max-bool-expr=5
 max-branches=15
 
 # Maximum number of locals for function / method body
-max-locals=20
+max-locals=25
 
 # Maximum number of parents for a class (see R0901).
 max-parents=7

dash/_callback.py

@@ -1,6 +1,7 @@
 import collections
 import hashlib
 from functools import wraps
+from typing import Callable, Optional, Any
 
 import flask
 
@@ -67,6 +68,7 @@ def callback(
     cancel=None,
     manager=None,
     cache_args_to_ignore=None,
+    on_error: Optional[Callable[[Exception], Any]] = None,
     **_kwargs,
 ):
     """
@@ -137,6 +139,10 @@ def callback(
             this should be a list of argument indices as integers.
         :param interval:
             Time to wait between the long callback update requests.
+        :param on_error:
+            Function to call when the callback raises an exception. Receives the
+            exception object as first argument. The callback_context can be used
+            to access the original callback inputs, states and output.
     """
 
     long_spec = None
@@ -186,6 +192,7 @@ def callback(
         long=long_spec,
         manager=manager,
         running=running,
+        on_error=on_error,
     )
 
 
@@ -226,7 +233,7 @@ def insert_callback(
     long=None,
     manager=None,
     running=None,
-    dynamic_creator=False,
+    dynamic_creator: Optional[bool] = False,
     no_output=False,
 ):
     if prevent_initial_call is None:
@@ -272,8 +279,16 @@ def insert_callback(
     return callback_id
 
 
-# pylint: disable=R0912, R0915
-def register_callback(  # pylint: disable=R0914
+def _set_side_update(ctx, response) -> bool:
+    side_update = dict(ctx.updated_props)
+    if len(side_update) > 0:
+        response["sideUpdate"] = side_update
+        return True
+    return False
+
+
+# pylint: disable=too-many-branches,too-many-statements
+def register_callback(
     callback_list, callback_map, config_prevent_initial_callbacks, *_args, **_kwargs
 ):
     (
@@ -297,6 +312,7 @@ def register_callback(  # pylint: disable=R0914
     long = _kwargs.get("long")
     manager = _kwargs.get("manager")
     running = _kwargs.get("running")
+    on_error = _kwargs.get("on_error")
     if running is not None:
         if not isinstance(running[0], (list, tuple)):
             running = [running]
@@ -342,6 +358,8 @@ def add_context(*args, **kwargs):
                 "callback_context", AttributeDict({"updated_props": {}})
             )
             callback_manager = long and long.get("manager", app_callback_manager)
+            error_handler = on_error or kwargs.pop("app_on_error", None)
+
             if has_output:
                 _validate.validate_output_spec(insert_output, output_spec, Output)
 
@@ -351,7 +369,7 @@ def add_context(*args, **kwargs):
                 args, inputs_state_indices
             )
 
-            response = {"multi": True}
+            response: dict = {"multi": True}
             has_update = False
 
             if long is not None:
@@ -440,10 +458,24 @@ def add_context(*args, **kwargs):
                     isinstance(output_value, dict)
                     and "long_callback_error" in output_value
                 ):
-                    error = output_value.get("long_callback_error")
-                    raise LongCallbackError(
+                    error = output_value.get("long_callback_error", {})
+                    exc = LongCallbackError(
                         f"An error occurred inside a long callback: {error['msg']}\n{error['tb']}"
                     )
+                    if error_handler:
+                        output_value = error_handler(exc)
+
+                        if output_value is None:
+                            output_value = NoUpdate()
+                        # set_props from the error handler uses the original ctx
+                        # instead of manager.get_updated_props since it runs in the
+                        # request process.
+                        has_update = (
+                            _set_side_update(callback_ctx, response)
+                            or output_value is not None
+                        )
+                    else:
+                        raise exc
 
                 if job_running and output_value is not callback_manager.UNDEFINED:
                     # cached results.
@@ -462,10 +494,22 @@ def add_context(*args, **kwargs):
                 if output_value is callback_manager.UNDEFINED:
                     return to_json(response)
             else:
-                output_value = _invoke_callback(func, *func_args, **func_kwargs)
-
-            if NoUpdate.is_no_update(output_value):
-                raise PreventUpdate
+                try:
+                    output_value = _invoke_callback(func, *func_args, **func_kwargs)
+                except PreventUpdate as err:
+                    raise err
+                except Exception as err:  # pylint: disable=broad-exception-caught
+                    if error_handler:
+                        output_value = error_handler(err)
+
+                        # If the error returns nothing, automatically puts NoUpdate for response.
+                        if output_value is None:
+                            if not multi:
+                                output_value = NoUpdate()
+                            else:
+                                output_value = [NoUpdate for _ in output_spec]
+                    else:
+                        raise err
 
             component_ids = collections.defaultdict(dict)
 
@@ -487,12 +531,12 @@ def add_context(*args, **kwargs):
                 )
 
                 for val, spec in zip(flat_output_values, output_spec):
-                    if isinstance(val, NoUpdate):
+                    if NoUpdate.is_no_update(val):
                         continue
                     for vali, speci in (
                         zip(val, spec) if isinstance(spec, list) else [[val, spec]]
                     ):
-                        if not isinstance(vali, NoUpdate):
+                        if not NoUpdate.is_no_update(vali):
                             has_update = True
                             id_str = stringify_id(speci["id"])
                             prop = clean_property_name(speci["property"])
@@ -506,10 +550,7 @@ def add_context(*args, **kwargs):
                 flat_output_values = []
 
             if not long:
-                side_update = dict(callback_ctx.updated_props)
-                if len(side_update) > 0:
-                    has_update = True
-                    response["sideUpdate"] = side_update
+                has_update = _set_side_update(callback_ctx, response) or has_update
 
             if not has_update:
                 raise PreventUpdate

dash/dash.py

@@ -16,7 +16,7 @@
 import base64
 import traceback
 from urllib.parse import urlparse
-from typing import Dict, Optional, Union
+from typing import Any, Callable, Dict, Optional, Union
 
 import flask
 
@@ -369,6 +369,10 @@ class Dash:
 
     :param description:  Sets a default description for meta tags on Dash pages (use_pages=True).
 
+    :param on_error: Global callback error handler to call when
+        an exception is raised. Receives the exception object as first argument.
+        The callback_context can be used to access the original callback inputs,
+        states and output.
     """
 
     _plotlyjs_url: str
@@ -409,6 +413,7 @@ def __init__(  # pylint: disable=too-many-statements
         hooks: Union[RendererHooks, None] = None,
         routing_callback_inputs: Optional[Dict[str, Union[Input, State]]] = None,
         description=None,
+        on_error: Optional[Callable[[Exception], Any]] = None,
         **obsolete,
     ):
         _validate.check_obsolete(obsolete)
@@ -520,6 +525,7 @@ def __init__(  # pylint: disable=too-many-statements
         self._layout = None
         self._layout_is_function = False
         self.validation_layout = None
+        self._on_error = on_error
         self._extra_components = []
 
         self._setup_dev_tools()
@@ -1377,6 +1383,7 @@ def dispatch(self):
                     outputs_list=outputs_list,
                     long_callback_manager=self._background_manager,
                     callback_context=g,
+                    app_on_error=self._on_error,
                 )
             )
         )

dash/long_callback/managers/celery_manager.py

@@ -161,6 +161,7 @@ def run():
             c.ignore_register_page = False
             c.updated_props = ProxySetProps(_set_props)
             context_value.set(c)
+            errored = False
             try:
                 if isinstance(user_callback_args, dict):
                     user_callback_output = fn(*maybe_progress, **user_callback_args)
@@ -170,13 +171,15 @@ def run():
                     user_callback_output = fn(*maybe_progress, user_callback_args)
             except PreventUpdate:
                 # Put NoUpdate dict directly to avoid circular imports.
+                errored = True
                 cache.set(
                     result_key,
                     json.dumps(
                         {"_dash_no_update": "_dash_no_update"}, cls=PlotlyJSONEncoder
                     ),
                 )
             except Exception as err:  # pylint: disable=broad-except
+                errored = True
                 cache.set(
                     result_key,
                     json.dumps(
@@ -188,7 +191,8 @@ def run():
                         },
                     ),
                 )
-            else:
+
+            if not errored:
                 cache.set(
                     result_key, json.dumps(user_callback_output, cls=PlotlyJSONEncoder)
                 )

dash/long_callback/managers/diskcache_manager.py

@@ -121,7 +121,8 @@ def call_job_fn(self, key, job_fn, args, context):
 
         # pylint: disable-next=not-callable
         proc = Process(
-            target=job_fn, args=(key, self._make_progress_key(key), args, context)
+            target=job_fn,
+            args=(key, self._make_progress_key(key), args, context),
         )
         proc.start()
         return proc.pid
@@ -187,6 +188,7 @@ def run():
             c.ignore_register_page = False
             c.updated_props = ProxySetProps(_set_props)
             context_value.set(c)
+            errored = False
             try:
                 if isinstance(user_callback_args, dict):
                     user_callback_output = fn(*maybe_progress, **user_callback_args)
@@ -195,8 +197,10 @@ def run():
                 else:
                     user_callback_output = fn(*maybe_progress, user_callback_args)
             except PreventUpdate:
+                errored = True
                 cache.set(result_key, {"_dash_no_update": "_dash_no_update"})
             except Exception as err:  # pylint: disable=broad-except
+                errored = True
                 cache.set(
                     result_key,
                     {
@@ -206,7 +210,8 @@ def run():
                         }
                     },
                 )
-            else:
+
+            if not errored:
                 cache.set(result_key, user_callback_output)
 
         ctx.run(run)

tests/integration/callbacks/test_callback_error.py

@@ -0,0 +1,43 @@
+from dash import Dash, html, Input, Output, set_props
+
+
+def test_cber001_error_handler(dash_duo):
+    def global_callback_error_handler(err):
+        set_props("output-global", {"children": f"global: {err}"})
+
+    app = Dash(on_error=global_callback_error_handler)
+
+    app.layout = [
+        html.Button("start", id="start-local"),
+        html.Button("start-global", id="start-global"),
+        html.Div(id="output"),
+        html.Div(id="output-global"),
+    ]
+
+    def on_callback_error(err):
+        set_props("output", {"children": f"callback: {err}"})
+
+    @app.callback(
+        Output("output", "children"),
+        Input("start-local", "n_clicks"),
+        on_error=on_callback_error,
+        prevent_initial_call=True,
+    )
+    def on_start(_):
+        raise Exception("local error")
+
+    @app.callback(
+        Output("output-global", "children"),
+        Input("start-global", "n_clicks"),
+        prevent_initial_call=True,
+    )
+    def on_start_global(_):
+        raise Exception("global error")
+
+    dash_duo.start_server(app)
+    dash_duo.find_element("#start-local").click()
+
+    dash_duo.wait_for_text_to_equal("#output", "callback: local error")
+
+    dash_duo.find_element("#start-global").click()
+    dash_duo.wait_for_text_to_equal("#output-global", "global: global error")

tests/integration/long_callback/app_bg_on_error.py

@@ -0,0 +1,50 @@
+from dash import Dash, Input, Output, html, set_props
+from tests.integration.long_callback.utils import get_long_callback_manager
+
+long_callback_manager = get_long_callback_manager()
+handle = long_callback_manager.handle
+
+
+def global_error_handler(err):
+    set_props("global-output", {"children": f"global: {err}"})
+
+
+app = Dash(
+    __name__, long_callback_manager=long_callback_manager, on_error=global_error_handler
+)
+
+app.layout = [
+    html.Button("callback on_error", id="start-cb-onerror"),
+    html.Div(id="cb-output"),
+    html.Button("global on_error", id="start-global-onerror"),
+    html.Div(id="global-output"),
+]
+
+
+def callback_on_error(err):
+    set_props("cb-output", {"children": f"callback: {err}"})
+
+
+@app.callback(
+    Output("cb-output", "children"),
+    Input("start-cb-onerror", "n_clicks"),
+    prevent_initial_call=True,
+    background=True,
+    on_error=callback_on_error,
+)
+def on_click(_):
+    raise Exception("callback error")
+
+
+@app.callback(
+    Output("global-output", "children"),
+    Input("start-global-onerror", "n_clicks"),
+    prevent_initial_call=True,
+    background=True,
+)
+def on_click_global(_):
+    raise Exception("global error")
+
+
+if __name__ == "__main__":
+    app.run(debug=True)