FastAPI示例代码启动触发TypeError错误，求解决方法

首次使用FastAPI，采用官方文档中的示例代码：

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
async def root():
    return {"message": "Hello World"}

执行前已安装所需依赖包：

pip install "fastapi[all]" 
pip install uvicorn

但执行启动命令uvicorn model_sm:app --reload时出现TypeError错误，错误信息如下：

INFO:     Will watch for changes in these directories: ['/Users/name/workplace/flask']
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     Started reloader process [24303] using StatReload
Process SpawnProcess-1:
Traceback (most recent call last):
  File "/usr/local/Cellar/python@3.9/3.9.15/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/process.py", line 315, in _bootstrap
    self.run()
  File "/usr/local/Cellar/python@3.9/3.9.15/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/local/lib/python3.9/site-packages/uvicorn/_subprocess.py", line 76, in subprocess_started
    target(sockets=sockets)
  File "/usr/local/lib/python3.9/site-packages/uvicorn/server.py", line 59, in run
    return asyncio.run(self.serve(sockets=sockets))
  File "/usr/local/Cellar/python@3.9/3.9.15/Frameworks/Python.framework/Versions/3.9/lib/python3.9/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/usr/local/Cellar/python@3.9/3.9.15/Frameworks/Python.framework/Versions/3.9/lib/python3.9/asyncio/base_events.py", line 647, in run_until_complete
    return future.result()
  File "/usr/local/lib/python3.9/site-packages/uvicorn/server.py", line 66, in serve
    config.load()
  File "/usr/local/lib/python3.9/site-packages/uvicorn/config.py", line 471, in load
    self.loaded_app = import_from_string(self.app)
  File "/usr/local/lib/python3.9/site-packages/uvicorn/importer.py", line 21, in import_from_string
    module = importlib.import_module(module_str)
  File "/usr/local/Cellar/python@3.9/3.9.15/Frameworks/Python.framework/Versions/3.9/lib/python3.9/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
  File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 680, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 850, in exec_module
  File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
  File "/Users/name/workplace/flask/model_sm.py", line 2, in <module>
    from fastapi import FastAPI
  File "/usr/local/lib/python3.9/site-packages/fastapi/__init__.py", line 7, in <module>
    from .applications import FastAPI as FastAPI
  File "/usr/local/lib/python3.9/site-packages/fastapi/applications.py", line 16, in <module>
    from fastapi import routing
  File "/usr/local/lib/python3.9/site-packages/fastapi/routing.py", line 478, in <module>
    class APIRouter(routing.Router):
  File "/usr/local/lib/python3.9/site-packages/fastapi/routing.py", line 497, in APIRouter
    lifespan: Optional[Lifespan[Any]] = None,
  File "/usr/local/Cellar/python@3.9/3.9.15/Frameworks/Python.framework/Versions/3.9/lib/python3.9/typing.py", line 277, in inner
    return func(*args, **kwds)
  File "/usr/local/Cellar/python@3.9/3.9.15/Frameworks/Python.framework/Versions/3.9/lib/python3.9/typing.py", line 768, in __getitem__
    _check_generic(self, params, len(self.__parameters__))
  File "/usr/local/lib/python3.9/site-packages/typing_extensions.py", line 97, in _check_generic
    raise TypeError(f"{cls} is not a generic class")
TypeError: typing.Union[typing.Callable[[ForwardRef('Starlette')], typing.AsyncContextManager[NoneType]], typing.Callable[[ForwardRef('Starlette')], typing.AsyncContextManager[typing.Mapping[str, typing.Any]]]] is not a generic class

这个错误根源是typing-extensions版本与Python 3.9不兼容。Python 3.9自带的typing模块和新版typing-extensions在泛型类型检查逻辑上存在冲突，而FastAPI新版本依赖的高版本typing-extensions触发了这个问题。

按以下步骤修复：

• 卸载当前typing-extensions：

  pip uninstall -y typing-extensions
  

• 安装兼容Python 3.9的旧版本（推荐4.7.1）：

  pip install typing-extensions==4.7.1
  

• 重新启动服务：

  uvicorn model_sm:app --reload
  

如果上述方法无效，可尝试降级FastAPI到兼容Python 3.9的版本（比如0.100.0）：

pip install fastapi==0.100.0

内容的提问来源于stack exchange，提问作者mohan