使用Uvicorn在Docker容器部署Flet应用时遇错求助

初始问题与错误 #

尝试在Docker容器中通过Uvicorn运行Flet Web应用时，遇到以下错误：

RuntimeError: asyncio.run() cannot be called from a running event loop
2024-05-27T18:46:46.450774857Z sys:1: RuntimeWarning: coroutine 'app_async' was never awaited

使用的启动代码：

def run_web():
    ft.app(target=main, export_asgi_app=True)

入口启动脚本：

#!/usr/bin/env bash

source .venv/bin/activate
uvicorn --factory reggui.main:run_web --host 0.0.0.0 --port 8060

更新后的问题与错误 #

将main改为异步函数后，应用可以启动，但访问时出现500错误，日志如下：

INFO:     Will watch for changes in these directories: ['/']
2024-05-28T11:57:22.142940833Z INFO:     Uvicorn running on http://0.0.0.0:8060 (Press CTRL+C to quit)
2024-05-28T11:57:22.142967464Z INFO:     Started reloader process [8] using WatchFiles
2024-05-28T11:57:22.899964258Z WARNING:  ASGI app factory detected. Using it, but please consider setting the --factory flag explicitly.
2024-05-28T11:57:22.900006979Z INFO:     Started server process [10]
2024-05-28T11:57:22.900026356Z INFO:     Waiting for application startup.
2024-05-28T11:57:22.900085509Z INFO:     ASGI 'lifespan' protocol appears unsupported.
2024-05-28T11:57:22.900107430Z INFO:     Application startup complete.
2024-05-28T11:57:53.724415871Z ERROR:    Exception in ASGI application
2024-05-28T11:57:53.724433625Z Traceback (most recent call last):
2024-05-28T11:57:53.724436010Z   File "/.venv/lib/python3.11/site-packages/uvicorn/protocols/http/httptools_impl.py", line 411, in run_asgi
2024-05-28T11:57:53.724438114Z     result = await app(  # type: ignore[func-returns-value]
2024-05-28T11:57:53.724439907Z              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2024-05-28T11:57:53.724441671Z   File "/.venv/lib/python3.11/site-packages/uvicorn/middleware/proxy_headers.py", line 69, in __call__
2024-05-28T11:57:53.724443514Z     return await self.app(scope, receive, send)
2024-05-28T11:57:53.724445257Z            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2024-05-28T11:57:53.724447001Z   File "/.venv/lib/python3.11/site-packages/uvicorn/middleware/asgi2.py", line 14, in __call__
2024-05-28T11:57:53.724448804Z     instance = self.app(scope)
2024-05-28T11:57:53.724450548Z                ^^^^^^^^^^^^^^^
2024-05-28T11:57:53.724452281Z TypeError: 'NoneType' object is not callable
2024-05-28T11:57:53.724445979Z INFO:     172.20.0.1:34188 - "GET / HTTP/1.1" 500 Internal Server Error
2024-05-28T11:57:53.745830886Z ERROR:    Exception in ASGI application
2024-05-28T11:57:53.745845213Z Traceback (most recent call last):
2024-05-28T11:57:53.745847708Z   File "/.venv/lib/python3.11/site-packages/uvicorn/protocols/http/httptools_impl.py", line 411, in run_asgi
2024-05-28T11:57:53.745849792Z     result = await app(  # type: ignore[func-returns-value]
2024-05-28T11:57:53.745851786Z              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2024-05-28T11:57:53.745853689Z   File "/.venv/lib/python3.11/site-packages/uvicorn/middleware/proxy_headers.py", line 69, in __call__
2024-05-28T11:57:53.745852287Z INFO:     172.20.0.1:34198 - "GET /favicon.ico HTTP/1.1" 500 Internal Server Error
2024-05-28T11:57:53.745855603Z     return await self.app(scope, receive, send)
2024-05-28T11:57:53.745871904Z            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2024-05-28T11:57:53.745874589Z   File "/.venv/lib/python3.11/site-packages/uvicorn/middleware/asgi2.py", line 14, in __call__
2024-05-28T11:57:53.745882965Z     instance = self.app(scope)
2024-05-28T11:57:53.745884188Z                ^^^^^^^^^^^^^^^
2024-05-28T11:57:53.745885290Z TypeError: 'NoneType' object is not callable

问题原因与解决方案 #

1. 初始错误原因 #

当使用ft.app(target=main, export_asgi_app=True)时，Flet会返回一个ASGI应用对象，但你的run_web()函数没有返回这个对象。同时，如果main是同步函数，Flet内部会调用asyncio.run()启动应用，而Uvicorn本身已经在运行事件循环，导致冲突报错。

2. 更新后错误原因 #

将main改为异步函数解决了事件循环冲突，但run_web()仍然没有返回ASGI应用对象，Uvicorn拿到的是None，当尝试调用这个None对象时就会触发TypeError: 'NoneType' object is not callable。

3. 最终解决方案 #

修改run_web()函数，让它返回Flet生成的ASGI应用：

import flet as ft

async def main(page: ft.Page):
    # 这里写你的应用逻辑
    page.title = "我的Flet应用"
    await page.add_async(ft.Text("Hello, Flet!"))

def run_web():
    # 返回ASGI应用对象
    return ft.app(target=main, export_asgi_app=True)

4. 额外优化建议 #

• Docker环境下如果不需要热重载，可以去掉Uvicorn的--reload参数（如果之前加了），避免不必要的文件监听；如果需要热重载，指定具体的代码目录，比如--reload-dir ./reggui，防止监听根目录/带来的性能问题。
• 确保Dockerfile中正确设置了工作目录，比如WORKDIR /app，并将代码复制到该目录下。
• 确认Docker容器的端口映射正确，比如docker run -p 8060:8060 your-image。

内容的提问来源于stack exchange，提问作者fccoelho