[BUG] dash.testing.ThreadedRunner misdetects a subclassed/wrapped FastAPI app as Flask (e.g. under OpenTelemetry auto-instrumentation) -> TypeError: Config.__init__() got an unexpected keyword argument 'threaded' #3942

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Describe your context

dash                                      4.4.0
fastapi                                   0.136.3
uvicorn                                   0.49.0

Also reproduced against dash 4.4.1 (latest release as of this report) — the relevant code in dash/testing/application_runners.py is unchanged between 4.4.0 and 4.4.1.

• OS: Windows 11
• Python: 3.14.5
• Not frontend-related (pure dash.testing / backend issue)

Describe the bug

dash.testing.application_runners.ThreadedRunner.start() picks the FastAPI/Quart vs. Flask app.run() branch by string-sniffing the server's class module:

module = app.server.__class__.__module__
# FastAPI support
if module.startswith("fastapi"):
    app.run(**options)
# Quart support (ASGI - runs its own async event loop)
elif module.startswith("quart"):
    app.run(**options)
# Flask fallback (WSGI - needs threaded mode)
else:
    app.run(threaded=True, **options)

This breaks for any fastapi.FastAPI subclass defined outside a module literally named fastapi.* — which is exactly what several common FastAPI instrumentation/wrapping libraries do, e.g. opentelemetry-instrumentation-fastapi's FastAPIInstrumentor, which replaces fastapi.FastAPI process-wide with opentelemetry.instrumentation.fastapi._InstrumentedFastAPI (a FastAPI subclass whose __module__ is "opentelemetry.instrumentation.fastapi", not "fastapi.*").

When that happens, ThreadedRunner falls through to the Flask branch and calls app.run(threaded=True, ...). For the FastAPI backend, that threaded kwarg flows straight into uvicorn.Config(...) (dash/backends/_fastapi.py), which doesn't accept it:

TypeError: Config.__init__() got an unexpected keyword argument 'threaded'

...which ThreadedRunner then reports as DashAppLoadingError: threaded server failed to start. This makes dash[testing] (and anything built on dash_duo/dash_thread_server, e.g. Playwright-based test suites using dash.testing.plugin) unusable for any FastAPI-backend app that has OpenTelemetry auto-instrumentation (or any other subclassing wrapper) applied to it.

Minimal reproduction (no OpenTelemetry install required — just simulates the same "FastAPI subclass defined in another module" shape):

import fastapi
from dash import Dash, html
from dash.testing.application_runners import ThreadedRunner


class WrappedFastAPI(fastapi.FastAPI):
    """Stands in for any third-party FastAPI subclass defined in another
    module -- e.g. opentelemetry.instrumentation.fastapi._InstrumentedFastAPI."""


WrappedFastAPI.__module__ = "some_other_package.wrapper"

server = WrappedFastAPI()
app = Dash(__name__, server=server)
app.layout = html.Div("hello")

runner = ThreadedRunner()
runner.start(app, start_timeout=5)  # raises DashAppLoadingError
print("started:", runner.started, runner.url)
runner.stop()

Real-world trigger, for reference:

from fastapi import FastAPI
from opentelemetry.instrumentation.fastapi import FastAPIInstrumentor
from dash import Dash

FastAPIInstrumentor().instrument()  # swaps fastapi.FastAPI -> _InstrumentedFastAPI

server = FastAPI()  # now actually an _InstrumentedFastAPI instance
app = Dash(__name__, server=server)
# any dash.testing fixture that goes through ThreadedRunner now fails

Expected behavior

ThreadedRunner should recognize a FastAPI (or Quart) app regardless of what module the concrete class is defined in — e.g. via isinstance(app.server, fastapi.FastAPI) / isinstance(app.server, quart.Quart) instead of a __module__ string prefix check. isinstance correctly handles subclasses defined anywhere, which a module-name string check fundamentally cannot.

Suggested fix in dash/testing/application_runners.py::ThreadedRunner.start():

try:
    import fastapi as _fastapi
except ImportError:
    _fastapi = None
try:
    import quart as _quart
except ImportError:
    _quart = None

if _fastapi is not None and isinstance(app.server, _fastapi.FastAPI):
    app.run(**options)
elif _quart is not None and isinstance(app.server, _quart.Quart):
    app.run(**options)
else:
    app.run(threaded=True, **options)

(MultiProcessRunner.start() in the same file has the identical pattern and would benefit from the same fix.)

Workaround we're using in the meantime (test-fixture-only, not a real fix): retag the instrumented class's __module__ before starting the server, e.g.

from opentelemetry.instrumentation.fastapi import _InstrumentedFastAPI
_InstrumentedFastAPI.__module__ = "fastapi.instrumented"

...but this is fragile (breaks if OTel's internal class name changes, and mutates third-party class state) and doesn't help anyone not already deep in dash.testing internals — hence this report.