Building support for decoupling Dash from Flask and supporting Quart / FastAPI servers #3430

BSd3v · 2025-09-09T12:36:29Z

This is an open PR draft, to contribute please target my forked branch.

The goal of this PR is to modularize the Dash setup to be independent of Flask (will fallback to Flask) and allow devs to configure their own backend.

decouple callback context from Flask Request
decouple index, assets, etc from build
build Quart server support
build FastAPI server support
allow for custom server build
add tests for servers

fixes #1571

work to modularize the dash eco-system and decouple from Flask

∙ - added types to BaseFactory to remove linting errors on create app in Flask and Quart Factory

… `request` context

…e flow. Made endpoints for downloading the reqs

* ∙ - remove contextvar from flask and quart only FastApi now relies on that ∙ - backend __init__ now holds the global request adapter and backend which get set on app initialisation ∙ request adapter and server can now be call from everywhere after the app initialised ∙ - added normal top level imports because the modules get matching loaded - but bad Import Error message when quart or equivilent are not installed ∙ - added _ as prefix to backends to avoid importing errors with their underlying ∙ - Can now move to remove unnecessary passing of the server object ∙ * Moved get_server_type to backends * ∙ moved async validation to validation ∙ replaced request.get_path with request.path ∙ * Moved custom backend check to _validation.py * Removed server injection of server methods - they use self.server now * removed use_async from dispatch server methods and use dash_app._use_async removed remaining set request process from flask * adding custom error handling per backend, tests and adjustments to the flow. Made endpoints for downloading the reqs * adjusments for formatting * adjustment to retest backend * Added Dash app as type to servers * adding missing reqs association * Addedd basic typing to servers * fixing minor linting issues * Fixed weird AI shit * Cleanup before heavy pull * Merged latest changes * f rebase * f rebase * Added Dash app as type to servers * Addedd basic typing to servers --------- Co-authored-by: Christian Giessel <cgiessel@tesla.com> Co-authored-by: BSd3v <82055130+BSd3v@users.noreply.github.com>

* ∙ Added has_request_context to base_server ∙ removed flask specific import in _validate and use backends.backend.has_request_context now * ∙ added context to request adapter ∙ callback context uses request adapter context now ∙ * added get_root_path to dash _utils removed flask.helpers.get_root_path usage * moved compress to server implementations flask fully decoupled from dash * fixed compress in quart and fastapi * Fixed server.config in fastapi to use config file * Update dash/backends/_fastapi.py Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com> * removed unused flask import in pages * Update dash/backends/_quart.py Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com> * removed flask specific return type to remove global dependency --------- Co-authored-by: Christian Giessel <cgiessel@tesla.com> Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

T4rk1n

This is looking good, a few fixes are needed and I left a few suggestion.

The serve_health route in dash.py still using flask.Response directly and need to be added in the backends.
There is a few Flask references left in comments on the Dash class that need to be changed.
The fastapi app is not running in debug mode if the app filename name is not "app.py".

T4rk1n · 2026-01-23T14:36:39Z

dash/_hooks.py

        """

-        def wrap(func: _t.Callable[[], _f.Response]):
+        def wrap(func: _t.Callable[[], _t.Any]):


This can be a future pain point to update existing hooks, maybe we can try adding a flask.Response converter in the adapter?

T4rk1n · 2026-01-23T14:51:36Z

dash/backends/__init__.py

+_backend_imports = {
+    "flask": ("dash.backends._flask", "FlaskDashServer"),
+    "fastapi": ("dash.backends._fastapi", "FastAPIDashServer"),
+    "quart": ("dash.backends._quart", "QuartDashServer"),
+}


I think we should have a way for this to be customizable, maybe add a hook for that. Then the backend library can have the hook inside it's init.

T4rk1n · 2026-01-23T14:52:54Z

dash/backends/_utils.py

+            <textarea readonly>{formatted_tb}</textarea>
+          </div>
+          <div class=\"explanation\">
+            The debugger caught an exception in your ASGI application. You can now


Suggested change

The debugger caught an exception in your ASGI application. You can now

The debugger caught an exception in your Dash application. You can now

T4rk1n · 2026-01-23T15:01:57Z

dash/backends/base_server.py

+    @property  # pragma: no cover - interface
+    @abstractmethod
+    def cookies(self):
+        raise NotImplementedError()
+
+    @property  # pragma: no cover - interface
+    @abstractmethod
+    def headers(self):
+        raise NotImplementedError()


Maybe we can type those as dict, then make sure the backends returns a dict copied from all the cookies/headers.

In the same way, maybe we can add a single get_cookie(...) and get_header(...) for single usage.

T4rk1n · 2026-01-23T15:03:39Z

dash/backends/base_server.py

+    @abstractmethod
+    def make_response(
+        self, data, mimetype=None, content_type=None
+    ) -> Any:  # pragma: no cover - interface
+        pass


The usual usecase for the response in the dash context is to set cookies/headers, maybe we should a global set_cookie(...) and set_header akin to set_props that can be used across the different backends with ease.

T4rk1n · 2026-01-23T17:24:31Z

dash/backends/_fastapi.py

+        if kwargs.get("reload"):
+            # Dynamically determine the module name from the file path
+            file_path = frame.filename
+            spec = spec_from_file_location("app", file_path)


We need the real file here, when I try with an app not named app with debug=True I get:

INFO: Uvicorn running on http://127.0.0.1:8050 (Press CTRL+C to quit) INFO: Started reloader process [80425] using StatReload ERROR: Error loading ASGI app. Could not import module "app". WARNING: StatReload detected changes in 'quart_app.py'. Reloading... ERROR: Error loading ASGI app. Could not import module "app".

T4rk1n · 2026-01-23T17:28:54Z

dash/backends/_fastapi.py

+            include_in_schema=False,
+        )
+
+    def dispatch(self, dash_app: Dash):


Might be a good time to rename this to a better name like serve_callback.

T4rk1n · 2026-01-23T17:34:00Z

dash/backends/_quart.py

+        if Response is None:
+            raise RuntimeError("Quart not installed; cannot generate Response")


This should not be happening, we should raise back the import error with a custom message to install dash[quart].

T4rk1n · 2026-01-23T17:34:58Z

dash/backends/_quart.py

+        return Response(
+            pkgutil.get_data("dash", "favicon.ico"), content_type="image/x-icon"
+        )


This is the same for all the backends, maybe it be in the base and just call the proper make_response to handle that?

T4rk1n · 2026-01-23T17:39:14Z

dash/backends/_quart.py

+    def serve_component_suites(
+        self, dash_app: Dash, package_name: str, fingerprinted_path: str
+    ):  # noqa: ARG002 unused req preserved for interface parity
+        path_in_pkg, has_fingerprint = check_fingerprint(fingerprinted_path)
+        _validate.validate_js_path(dash_app.registered_paths, package_name, path_in_pkg)
+        extension = "." + path_in_pkg.split(".")[-1]
+        mimetype = mimetypes.types_map.get(extension, "application/octet-stream")
+        package = sys.modules[package_name]
+        dash_app.logger.debug(
+            "serving -- package: %s[%s] resource: %s => location: %s",
+            package_name,
+            getattr(package, "__version__", "unknown"),
+            path_in_pkg,
+            package.__path__,
+        )
+        data = pkgutil.get_data(package_name, path_in_pkg)
+        headers = {}
+        if has_fingerprint:
+            headers["Cache-Control"] = "public, max-age=31536000"
+
+        if Response is None:
+            raise RuntimeError("Quart not installed; cannot generate Response")
+        return Response(data, content_type=mimetype, headers=headers)


This function will mostly be the same across backends with a difference in the Response type, in this case we are missing the etag stuff that might need to be adapted.

I think maybe we could have a Response type that is dash specific and then we have a ResponseAdapter like we do for the RequestAdapter.

BSd3v added 7 commits September 9, 2025 08:24

initial push

c457ab7

work to modularize the dash eco-system and decouple from Flask

4ebc657

fix favicon

9dff791

removing changelog entry

c319b18

fixing issue with debug true for FastAPI

7de2a41

fixing catchall for path routes

2cd769e

fixing pages for use with fastapi

686f32f

gvwilson assigned BSd3v Sep 10, 2025

gvwilson added feature something new P2 considered for next cycle community community contribution labels Sep 10, 2025

BSd3v and others added 19 commits September 10, 2025 10:04

fixing issue with flask pages

660e257

Merge pull request #3 from BSd3v/modularize-dash-server

581f8a5

work to modularize the dash eco-system and decouple from Flask

Merge branch 'dev' into bring-your-own-server

9eb9dd0

fixing for lint

0fa5c99

fixing issue with failing test due to endpoint name

1088331

fixing run command to trigger devtools properly

4920e33

fixing issue with lint and debug ui

9ffba5a

fixing issue with _app when using dispatch, need to keep in context

908aacd

fixing issue with catchall

9491c7f

fixing issue with args and cancelling callbacks

39ad7bd

fixing issues with pages metadata and flaky tests

7bf69a7

fixing issues with relativate paths

10681dc

∙ - initial quart factory

4944d6d

∙ - added types to BaseFactory to remove linting errors on create app in Flask and Quart Factory

Quart factory ready

3b0f47e

fixing for lint

1112f77

fixing issue with apps overwriting other paths

8c52bbb

removing print

aabeeb7

cleanup

5659cd7

reverting render_index -> index and making catch for outside of a…

b05e376

… `request` context

BSd3v and others added 18 commits September 16, 2025 16:18

adding custom error handling per backend, tests and adjustments to th…

8c78089

…e flow. Made endpoints for downloading the reqs

adjusments for formatting

5211f6f

adjustment to retest backend

6a34208

adding missing reqs association

1a2b531

fixing minor linting issues

465e45e

fixes for failing tests

c4795ed

fixing formatting

567d0f8

fixing issues

a855c6d

fixing async validation

79afb0b

adjustments for request_adapter

77e22a3

adding test for custom dash server

f7331d3

fixing issue with request_adapter

8b58cf4

adjusting error handling for fastapi

b7d4af2

adjustments for handling issues with debug for fastapi

4cf4686

fixing for lint

dfe0ac7

adjustment for delayed config

cd02cc5

fix typing error

16b3c9e

BSd3v marked this pull request as ready for review September 18, 2025 20:34

BSd3v requested review from T4rk1n, emilykl, gvwilson and ndrezn as code owners September 18, 2025 20:34

BSd3v and others added 6 commits September 22, 2025 08:30

fixes for pages

493d150

Merge branch 'dev' into bring-your-own-server

26f01b2

Merge branch 'dev' into bring-your-own-server

02e0be5

Merge branch 'dev' into bring-your-own-server

39b1096

Merge branch 'dev' into bring-your-own-server

acc0179

T4rk1n reviewed Jan 23, 2026

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Building support for decoupling Dash from Flask and supporting Quart / FastAPI servers #3430

Building support for decoupling Dash from Flask and supporting Quart / FastAPI servers #3430

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	The debugger caught an exception in your ASGI application. You can now
	The debugger caught an exception in your Dash application. You can now

		if Response is None:
		raise RuntimeError("Quart not installed; cannot generate Response")

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Building support for decoupling Dash from Flask and supporting Quart / FastAPI servers #3430

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BSd3v commented Sep 9, 2025 •

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