Add app_context to get_app, allowing to get the current app in routes

CHANGELOG.md

@@ -6,6 +6,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 
 ## Added
 - [#3395](https://github.com/plotly/dash/pull/3396) Add position argument to hooks.devtool
+- [#3403](https://github.com/plotly/dash/pull/3403) Add app_context to get_app, allowing to get the current app in routes.
 
 ## Fixed
 - [#3395](https://github.com/plotly/dash/pull/3395) Fix Components added through set_props() cannot trigger related callback functions. Fix [#3316](https://github.com/plotly/dash/issues/3316)

dash/_get_app.py

@@ -1,16 +1,58 @@
+import functools
+
+from contextvars import ContextVar, copy_context
 from textwrap import dedent
 
 APP = None
 
+app_context = ContextVar("dash_app_context")
+
+
+def with_app_context(func):
+    @functools.wraps(func)
+    def wrap(self, *args, **kwargs):
+        app_context.set(self)
+        ctx = copy_context()
+        return ctx.run(func, self, *args, **kwargs)
+
+    return wrap
+
+
+def with_app_context_async(func):
+    @functools.wraps(func)
+    async def wrap(self, *args, **kwargs):
+        app_context.set(self)
+        ctx = copy_context()
+        print("copied and set")
+        return await ctx.run(func, self, *args, **kwargs)
+
+    return wrap
+
+
+def with_app_context_factory(func, app):
+    @functools.wraps(func)
+    def wrap(*args, **kwargs):
+        app_context.set(app)
+        ctx = copy_context()
+        return ctx.run(func, *args, **kwargs)
+
+    return wrap
+
 
 def get_app():
+    try:
+        ctx_app = app_context.get()
+        if ctx_app is not None:
+            return ctx_app
+    except LookupError:
+        pass
+
     if APP is None:
         raise Exception(
             dedent(
                 """
                 App object is not yet defined.  `app = dash.Dash()` needs to be run
-                before `dash.get_app()` is called and can only be used within apps that use
-                the `pages` multi-page app feature: `dash.Dash(use_pages=True)`.
+                before `dash.get_app()`.
 
                 `dash.get_app()` is used to get around circular import issues when Python files
                 within the pages/` folder need to reference the `app` object.

dash/dash.py

@@ -68,6 +68,7 @@
 from . import _watch
 from . import _get_app
 
+from ._get_app import with_app_context, with_app_context_async, with_app_context_factory
 from ._grouping import map_grouping, grouping_len, update_args_group
 from ._obsolete import ObsoleteChecker
 
@@ -773,7 +774,11 @@ def _setup_routes(self):
             )
 
         for hook in self._hooks.get_hooks("routes"):
-            self._add_url(hook.data["name"], hook.func, hook.data["methods"])
+            self._add_url(
+                hook.data["name"],
+                with_app_context_factory(hook.func, self),
+                hook.data["methods"],
+            )
 
         # catch-all for front-end routes, used by dcc.Location
         self._add_url("<path:path>", self.index)
@@ -840,6 +845,7 @@ def index_string(self, value: str) -> None:
         _validate.validate_index("index string", checks, value)
         self._index_string = value
 
+    @with_app_context
     def serve_layout(self):
         layout = self._layout_value()
 
@@ -1143,6 +1149,7 @@ def serve_component_suites(self, package_name, fingerprinted_path):
 
         return response
 
+    @with_app_context
     def index(self, *args, **kwargs):  # pylint: disable=unused-argument
         scripts = self._generate_scripts_html()
         css = self._generate_css_dist_html()
@@ -1256,6 +1263,7 @@ def interpolate_index(self, **kwargs):
             app_entry=app_entry,
         )
 
+    @with_app_context
     def dependencies(self):
         return flask.Response(
             to_json(self._callback_list),
@@ -1464,6 +1472,7 @@ def _execute_callback(self, func, args, outputs_list, g):
         )
         return partial_func
 
+    @with_app_context_async
     async def async_dispatch(self):
         body = flask.request.get_json()
         g = self._initialize_context(body)
@@ -1483,6 +1492,7 @@ async def async_dispatch(self):
         g.dash_response.set_data(response_data)
         return g.dash_response
 
+    @with_app_context
     def dispatch(self):
         body = flask.request.get_json()
         g = self._initialize_context(body)
@@ -1833,7 +1843,11 @@ def setup_startup_routes(self) -> None:
         Initialize the startup routes stored in STARTUP_ROUTES.
         """
         for _name, _view_func, _methods in self.STARTUP_ROUTES:
-            self._add_url(f"_dash_startup_route/{_name}", _view_func, _methods)
+            self._add_url(
+                f"_dash_startup_route/{_name}",
+                with_app_context_factory(_view_func, self),
+                _methods,
+            )
         self.STARTUP_ROUTES = []
 
     def _setup_dev_tools(self, **kwargs):

tests/integration/test_hooks.py

@@ -2,7 +2,7 @@
 import requests
 import pytest
 
-from dash import Dash, Input, Output, html, hooks, set_props, ctx
+from dash import Dash, Input, Output, html, hooks, set_props, ctx, get_app
 
 
 @pytest.fixture
@@ -240,3 +240,103 @@ def cb(_):
     )
     dash_duo.wait_for_element("#devtool").click()
     dash_duo.wait_for_text_to_equal("#output", "hooked from devtools")
+
+
+def test_hook012_get_app_available_in_hooks_on_routes(hook_cleanup, dash_duo):
+    """Test that get_app() is available during hooks when @with_app_context decorated routes are called."""
+
+    # Track which hooks were able to access get_app()
+    hook_access_results = {
+        "layout_hook": False,
+        "error_hook": False,
+        "callback_hook": False,
+    }
+
+    @hooks.layout()
+    def layout_hook(layout):
+        try:
+            retrieved_app = get_app()
+            hook_access_results["layout_hook"] = retrieved_app is not None
+        except Exception:
+            hook_access_results["layout_hook"] = False
+        return layout
+
+    @hooks.error()
+    def error_hook(error):
+        try:
+            retrieved_app = get_app()
+            hook_access_results["error_hook"] = retrieved_app is not None
+        except Exception:
+            hook_access_results["error_hook"] = False
+
+    @hooks.callback(
+        Output("hook-output", "children"),
+        Input("hook-button", "n_clicks"),
+        prevent_initial_call=True,
+    )
+    def callback_hook(n_clicks):
+        try:
+            retrieved_app = get_app()
+            hook_access_results["callback_hook"] = retrieved_app is not None
+            return f"Hook callback: {n_clicks}"
+        except Exception as err:
+            hook_access_results["callback_hook"] = False
+            return f"Error in hook callback: {err}"
+
+    app = Dash(__name__)
+    app.layout = [
+        html.Div("Test get_app in hooks", id="main"),
+        html.Button("Click for callback", id="button"),
+        html.Div(id="output"),
+        html.Button("Hook callback", id="hook-button"),
+        html.Div(id="hook-output"),
+        html.Button("Error", id="error-btn"),
+        html.Div(id="error-output"),
+    ]
+
+    @app.callback(
+        Output("output", "children"),
+        Input("button", "n_clicks"),
+        prevent_initial_call=True,
+    )
+    def test_callback(n_clicks):
+        return f"Clicked {n_clicks} times"
+
+    @app.callback(
+        Output("error-output", "children"),
+        Input("error-btn", "n_clicks"),
+        prevent_initial_call=True,
+    )
+    def error_callback(n_clicks):
+        if n_clicks:
+            raise ValueError("Test error for hook")
+        return ""
+
+    dash_duo.start_server(app)
+
+    # Test the @with_app_context decorated routes
+
+    # 2. Test layout hook via index route (GET /)
+    dash_duo.wait_for_text_to_equal("#main", "Test get_app in hooks")
+
+    # 3. Test callback hook via dispatch route (POST /_dash-update-component)
+    dash_duo.wait_for_element("#hook-button").click()
+    dash_duo.wait_for_text_to_equal("#hook-output", "Hook callback: 1")
+
+    # 4. Test error hook via dispatch route when error occurs
+    dash_duo.wait_for_element("#error-btn").click()
+    # Give error hook time to execute
+    import time
+
+    time.sleep(0.5)
+
+    # Verify that get_app() worked in hooks during route calls with @with_app_context
+    assert hook_access_results[
+        "layout_hook"
+    ], "get_app() should be accessible in layout hook when serve_layout/index routes have @with_app_context"
+    assert hook_access_results[
+        "callback_hook"
+    ], "get_app() should be accessible in callback hook when dispatch route has @with_app_context"
+    assert hook_access_results[
+        "error_hook"
+    ], "get_app() should be accessible in error hook when dispatch route has @with_app_context"