Fix background callbacks & register page

dash/_pages.py

@@ -9,6 +9,7 @@
 from . import _validate
 from ._utils import AttributeDict
 from ._get_paths import get_relative_path
+from ._callback_context import context_value
 
 
 CONFIG = AttributeDict()
@@ -250,6 +251,9 @@ def register_page(
     ])
     ```
     """
+    if context_value.get().get("ignore_register_page"):
+        return
+
     _validate.validate_use_pages(CONFIG)
 
     page = dict(

dash/_validate.py

@@ -8,6 +8,7 @@
 from .development.base_component import Component
 from . import exceptions
 from ._utils import patch_collections_abc, stringify_id, to_json, coerce_to_list
+from .exceptions import PageError
 
 
 def validate_callback(outputs, inputs, state, extra_args, types):
@@ -462,10 +463,10 @@ def validate_pages_layout(module, page):
 
 def validate_use_pages(config):
     if not config.get("assets_folder", None):
-        raise Exception("`dash.register_page()` must be called after app instantiation")
+        raise PageError("`dash.register_page()` must be called after app instantiation")
 
     if flask.has_request_context():
-        raise Exception(
+        raise PageError(
             """
             dash.register_page() can’t be called within a callback as it updates dash.page_registry, which is a global variable.
              For more details, see https://dash.plotly.com/sharing-data-between-callbacks#why-global-variables-will-break-your-app

dash/dash.py

@@ -1208,6 +1208,8 @@ def dispatch(self):
             cb = self.callback_map[output]
             func = cb["callback"]
 
+            g.ignore_register_page = cb.get("long", False)
+
             # Add args_grouping
             inputs_state_indices = cb["inputs_state_indices"]
             inputs_state = inputs + state

dash/exceptions.py

@@ -93,3 +93,7 @@ class LongCallbackError(DashException):
 
 class MissingLongCallbackManagerError(DashException):
     pass
+
+
+class PageError(DashException):
+    pass

dash/testing/__init__.py

@@ -0,0 +1,17 @@
+from contextlib import contextmanager
+
+from .._callback_context import context_value as _ctx
+from .._utils import AttributeDict as _AD
+
+
+@contextmanager
+def ignore_register_page():
+    previous = _ctx.get()
+    copied = _AD(previous)
+    copied.ignore_register_page = True
+    _ctx.set(copied)
+
+    try:
+        yield
+    finally:
+        _ctx.set(previous)

tests/unit/test_testing.py

@@ -0,0 +1,7 @@
+from dash.testing import ignore_register_page
+from dash import register_page
+
+
+def test_tst001_ignore_register_page():
+    with ignore_register_page():
+        register_page("/")