bug: Example Fastapi | Add flet to existing fastapi app --> missing 1 required positional argument: 'before_main'

[NiTRoeSE]

Duplicate Check

• I have searched the opened issues and there are no duplicates

Describe the bug

Hello,

the example https://docs.flet.dev/publish/web/dynamic-website/#hosting-multiple-flet-apps-under-the-same-domain didnt work.

I always get the error TypeError: app() missing 1 required positional argument: 'before_main'.

Hope someone can help or fix.
Thanks in advanced

Code sample

Code

from contextlib import asynccontextmanager

import flet as ft
import flet.fastapi as flet_fastapi
from fastapi import FastAPI

@asynccontextmanager
async def lifespan(app: FastAPI):
    await flet_fastapi.app_manager.start()
    yield
    await flet_fastapi.app_manager.shutdown()

app = FastAPI(lifespan=lifespan)

async def main(page: ft.Page):
    page.add(ft.Text("Hello, Flet!"))

app.mount("/flet-app", flet_fastapi.app(main))

To reproduce

run example colde via uvicorn main:app --host localhost --port 8550

Expected behavior

Should work...

Screenshots / Videos

Captures

[Upload media here]

Operating System

macOS

Operating system details

26.2

Flet version

0.80.5

Regression

No, it isn't

Suggestions

I dont know..

Logs

Logs

INFO:     Uvicorn running on http://localhost:8550 (Press CTRL+C to quit)
INFO:     Started parent process [48809]
Process SpawnProcess-4:
Process SpawnProcess-2:
Process SpawnProcess-1:
Process SpawnProcess-3:
Traceback (most recent call last):
Traceback (most recent call last):
Traceback (most recent call last):
  File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/process.py", line 313, in _bootstrap
    self.run()
    ~~~~~~~~^^
  File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/process.py", line 313, in _bootstrap
    self.run()
    ~~~~~~~~^^
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/_subprocess.py", line 80, in subprocess_started
    target(sockets=sockets)
    ~~~~~~^^^^^^^^^^^^^^^^^
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/supervisors/multiprocess.py", line 64, in target
    return self.real_targ  File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
et(sockets)
           ~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/_subprocess.py", line 80, in subprocess_started
    target(sockets=sockets)
    ~~~~~~^^^^^^^^^^^^^^^^^
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/supervisors/multiprocess.py", line 64, in target
    return self.real_target(sockets)
           ~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/process.py", line 313, in _bootstrap
    self.run()
    ~~~~~~~~^^
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/server.py", line 67, in run
    return asyncio_run(self.serve(sockets=sockets), loop_factory=self.config.get_loop_factory())
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/server.py", line 67, in run
    return asyncio_run(self.serve(sockets=sockets), loop_factory=self.config.get_loop_factory())
  File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/asyncio/runners.py", line 195, in run
    return runner.run(main)
           ~~~~~~~~~~^^^^^^
  File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/_subprocess.py", line 80, in subprocess_started
    target(sockets=sockets)
    ~~~~~~^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/asyncio/runners.py", line 195, in run
    return runner.run(main)
           ~~~~~~~~~~^^^^^^
Traceback (most recent call last):
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/supervisors/multiprocess.py", line 64, in target
    return self.real_target(sockets)
           ~~~~~~~~~~~~~~~~^^^^^^^^^
  File "uvloop/loop.pyx", line 1518, in uvloop.loop.Loop.run_until_complete
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/server.py", line 67, in run
    return asyncio_run(self.serve(sockets=sockets), loop_factory=self.config.get_loop_factory())
  File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/server.py", line 71, in serve
    await self._serve(sockets)
  File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/asyncio/runners.py", line 195, in run
    return runner.run(main)
           ~~~~~~~~~~^^^^^^
  File "uvloop/loop.pyx", line 1518, in uvloop.loop.Loop.run_until_complete
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/server.py", line 78, in _serve
    config.load()
    ~~~~~~~~~~~^^
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/server.py", line 71, in serve
    await self._serve(sockets)
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/config.py", line 439, in load
    self.loaded_app = import_from_string(self.app)
                      ~~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "uvloop/loop.pyx", line 1518, in uvloop.loop.Loop.run_until_complete
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/importer.py", line 19, in import_from_string
    module = importlib.import_module(module_str)
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/server.py", line 71, in serve
    await self._serve(sockets)
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/server.py", line 78, in _serve
    config.load()
    ~~~~~~~~~~~^^
  File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/importlib/__init__.py", line 88, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/server.py", line 78, in _serve
    config.load()
    ~~~~~~~~~~~^^
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/config.py", line 439, in load
    self.loaded_app = import_from_string(self.app)
                      ~~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "<frozen importlib._bootstrap>", line 1387, in _gcd_import
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/config.py", line 439, in load
    self.loaded_app = import_from_string(self.app)
                      ~~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/importer.py", line 19, in import_from_string
    module = importlib.import_module(module_str)
  File "<frozen importlib._bootstrap>", line 1360, in _find_and_load
  File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/importlib/__init__.py", line 88, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/importer.py", line 19, in import_from_string
    module = importlib.import_module(module_str)
  File "<frozen importlib._bootstrap>", line 1331, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 935, in _load_unlocked
  File "<frozen importlib._bootstrap>", line 1387, in _gcd_import
  File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/importlib/__init__.py", line 88, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen importlib._bootstrap_external>", line 1026, in exec_module
  File "<frozen importlib._bootstrap>", line 1360, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1387, in _gcd_import
  File "<frozen importlib._bootstrap>", line 488, in _call_with_frames_removed
  File "<frozen importlib._bootstrap>", line 1331, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 1360, in _find_and_load
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/src/test.py", line 18, in <module>
    app.mount("/flet-app", flet_fastapi.app(main))
                           ~~~~~~~~~~~~~~~~^^^^^^
  File "<frozen importlib._bootstrap>", line 935, in _load_unlocked
  File "<frozen importlib._bootstrap>", line 1331, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap_external>", line 1026, in exec_module
  File "<frozen importlib._bootstrap>", line 935, in _load_unlocked
  File "<frozen importlib._bootstrap>", line 488, in _call_with_frames_removed
  File "<frozen importlib._bootstrap_external>", line 1026, in exec_module
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/src/test.py", line 18, in <module>
    app.mount("/flet-app", flet_fastapi.app(main))
                           ~~~~~~~~~~~~~~~~^^^^^^
  File "<frozen importlib._bootstrap>", line TypeError: app() missing 1 required positional argument: 'before_main'
TypeError: app() missing 1 required positional argument: 'before_main'
488, in _call_with_frames_removed
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/src/test.py", line 18, in <module>
    app.mount("/flet-app", flet_fastapi.app(main))
                           ~~~~~~~~~~~~~~~~^^^^^^
TypeError: app() missing 1 required positional argument: 'before_main'
  File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/process.py", line 313, in _bootstrap
    self.run()
    ~~~~~~~~^^
  File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/_subprocess.py", line 80, in subprocess_started
    target(sockets=sockets)
    ~~~~~~^^^^^^^^^^^^^^^^^
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/supervisors/multiprocess.py", line 64, in target
    return self.real_target(sockets)
           ~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/server.py", line 67, in run
    return asyncio_run(self.serve(sockets=sockets), loop_factory=self.config.get_loop_factory())
  File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/asyncio/runners.py", line 195, in run
    return runner.run(main)
           ~~~~~~~~~~^^^^^^
  File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "uvloop/loop.pyx", line 1518, in uvloop.loop.Loop.run_until_complete
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/server.py", line 71, in serve
    await self._serve(sockets)
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/server.py", line 78, in _serve
    config.load()
    ~~~~~~~~~~~^^
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/config.py", line 439, in load
    self.loaded_app = import_from_string(self.app)
                      ~~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/venv/lib/python3.13/site-packages/uvicorn/importer.py", line 19, in import_from_string
    module = importlib.import_module(module_str)
  File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/importlib/__init__.py", line 88, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen importlib._bootstrap>", line 1387, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1360, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1331, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 935, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 1026, in exec_module
  File "<frozen importlib._bootstrap>", line 488, in _call_with_frames_removed
  File "/Users/nitro/Medialyse-Git/bbb-recording-downloader-flet/src/test.py", line 18, in <module>
    app.mount("/flet-app", flet_fastapi.app(main))
                           ~~~~~~~~~~~~~~~~^^^^^^
TypeError: app() missing 1 required positional argument: 'before_main'

Additional details

I dont know whats the problem, i need fastapi because that seems the only way to downloading a file from flet web app.

Maybe there is another way to download files but i founf nothing in documentation.

[ndonkoHenri]

Thanks for reporting. Please try the latest release (v0.81.0), which doesnt have this issue.