Add optional callbacks

dash/_callback.py

@@ -77,6 +77,7 @@ def callback(
     cache_args_to_ignore: Optional[list] = None,
     cache_ignore_triggered=True,
     on_error: Optional[Callable[[Exception], Any]] = None,
+    optional: Optional[bool] = False,
     **_kwargs,
 ) -> Callable[..., Any]:
     """
@@ -159,6 +160,8 @@ def callback(
             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.
+        :param optional:
+            Mark all dependencies as not required on the initial layout checks.
     """
 
     background_spec = None
@@ -213,6 +216,7 @@ def callback(
         manager=manager,
         running=running,
         on_error=on_error,
+        optional=optional,
     )
 
 
@@ -258,6 +262,7 @@ def insert_callback(
     running=None,
     dynamic_creator: Optional[bool] = False,
     no_output=False,
+    optional=False,
 ):
     if prevent_initial_call is None:
         prevent_initial_call = config_prevent_initial_callbacks
@@ -281,6 +286,7 @@ def insert_callback(
         },
         "dynamic_creator": dynamic_creator,
         "no_output": no_output,
+        "optional": optional,
     }
     if running:
         callback_spec["running"] = running
@@ -624,6 +630,7 @@ def register_callback(
         dynamic_creator=allow_dynamic_callbacks,
         running=running,
         no_output=not has_output,
+        optional=_kwargs.get("optional", False),
     )
 
     # pylint: disable=too-many-locals

dash/dash-renderer/src/actions/dependencies.js

@@ -570,12 +570,18 @@ export function validateCallbacksToLayout(state_, dispatchError) {
         for (const id in map) {
             const idProps = map[id];
             const fcb = flatten(values(idProps));
-            const optional = all(
-                ({allow_optional}) => allow_optional,
-                flatten(
-                    fcb.map(cb => concat(cb.outputs, cb.inputs, cb.states))
-                ).filter(dep => dep.id === id)
-            );
+            const optional = fcb.reduce((acc, cb) => {
+                if (acc === false || cb.optional) {
+                    return acc;
+                }
+                const deps = concat(cb.outputs, cb.inputs, cb.states).filter(
+                    dep => dep.id === id
+                );
+                return (
+                    !deps.length ||
+                    all(({allow_optional}) => allow_optional, deps)
+                );
+            }, true);
             if (optional) {
                 continue;
             }