[BUG]: Fastapi page routes trigger 500 internal server error

[joschrag]

Thank you so much for helping improve the quality of Dash!

We do our best to catch bugs during the release process, but we rely on your help to find the ones that slip through.

Describe your context
Please provide us your environment, so we can easily reproduce the issue.

dash 4.3.0

Describe the bug

When creating a simple dash app with fastapi backend like this:

app.py

import dash
from dash import Dash, html

app = Dash(__name__, backend="fastapi", use_pages=True)
app.layout = html.Div([html.H1("MWE"), dash.page_container])

if __name__ == "__main__":
 app.run()

pages/index.py

import dash
from dash import html

dash.register_page(__name__, path="/", layout=html.Div("home"))

pages/home.py

import dash
from dash import html

dash.register_page(__name__, path="/home", layout=html.Div("home"))

When browsing these pages or with curl we get the following:

curl -w "%{http_code}" -o NUL -s http://127.0.0.1:8050/ -> 200
curl -w "%{http_code}" -o NUL -s http://127.0.0.1:8050/home -> 500

with the traceback:

Traceback (most recent call last):
 File "...\uvicorn\protocols\http\httptools_impl.py", line 421, in run_asgi
 result = await app( # type: ignore[func-returns-value]
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "...\uvicorn\middleware\proxy_headers.py", line 62, in __call__
 return await self.app(scope, receive, send)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "...\fastapi\applications.py", line 1159, in __call__
 await super().__call__(scope, receive, send)
 File "...\starlette\applications.py", line 90, in __call__
 await self.middleware_stack(scope, receive, send)
 File "...\starlette\middleware\errors.py", line 186, in __call__
 raise exc
 File "...\starlette\middleware\errors.py", line 164, in __call__
 await self.app(scope, receive, _send)
 File "...\dash\backends\_fastapi.py", line 235, in __call__
 await self.app(scope, receive, send)
 File "...\starlette\middleware\exceptions.py", line 63, in __call__
 await wrap_app_handling_exceptions(self.app, conn)(scope, receive, send)
 File "...\starlette\_exception_handler.py", line 53, in wrapped_app
 raise exc
 File "...\starlette\_exception_handler.py", line 42, in wrapped_app
 await app(scope, receive, sender)
 File "...\fastapi\middleware\asyncexitstack.py", line 18, in __call__
 await self.app(scope, receive, send)
 File "...\starlette\routing.py", line 660, in __call__
 await self.middleware_stack(scope, receive, send)
 File "...\starlette\routing.py", line 680, in app
 await route.handle(scope, receive, send)
 File "...\starlette\routing.py", line 276, in handle
 await self.app(scope, receive, send)
 File "...\fastapi\routing.py", line 134, in app
 await wrap_app_handling_exceptions(app, request)(scope, receive, send)
 File "...\starlette\_exception_handler.py", line 53, in wrapped_app
 raise exc
 File "...\starlette\_exception_handler.py", line 42, in wrapped_app
 await app(scope, receive, sender)
 File "...\fastapi\routing.py", line 120, in app
 response = await f(request)
 ^^^^^^^^^^^^^^^^
 File "...\fastapi\routing.py", line 674, in app
 raw_response = await run_endpoint_function(
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "...\fastapi\routing.py", line 328, in run_endpoint_function
 return await dependant.call(**values)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "...\dash\backends\_fastapi.py", line 338, in catchall
 return Response(content=dash_app.index(), media_type="text/html")
 ^^^^^^^^^^^^^^^^
 File "...\dash\dash.py", line 1302, in index
 request = self.backend.request_adapter()
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "...\dash\backends\_fastapi.py", line 842, in __init__
 self._request: Request = get_current_request()
 ^^^^^^^^^^^^^^^^^^^^^
 File "...\dash\backends\_fastapi.py", line 109, in get_current_request
 raise RuntimeError("No active request in context")
RuntimeError: No active request in context

#3805 seems to be the cause as the /home route does not match any conditions and exits before the request context is set in dash/backends/_fastapi.py

async def __call_():
...

 if (
 not path.startswith(dash_prefix)
 and path != prefix
 and path != prefix.rstrip("/")
 ):
 await self.app(scope, receive, send) # /home fails within here
 return
 
 request = Request(scope, receive=receive)
 token = set_current_request(request)
...

Expected behavior

The /home path should return a 200 Response and its content.

Screenshots

[camdecoster]

Thanks for the bug report! Our team will take a look.

[rgajason]

This looks to be a duplicate of #3812, which just had a fix merged.

You may wish to check out #3814 as well (different scenario, but similar issue).

[joschrag]

This looks to be a duplicate of #3812, which just had a fix merged.

You may wish to check out #3814 as well (different scenario, but similar issue).

Thank you for bringing this up. I just tested against the newest dev branch and the issue has been fixed via #3812 (#3819)