ImportError: cannot import name 'ContextVar' from 'werkzeug.local'

[zxCoder]

flask 2.0.1

一个小时前 flask 还正常，后来装了几个其他包，再启动就报错了

ImportError: cannot import name 'ContextVar' from 'werkzeug.local'

定位在

from werkzeug.local import ContextVar

可是 werkzeug/local.py 这个文件里确实没有 ContextVar

是哪里出了问题呢？ pip 重装了也不行

[killva4624]

看看你的 import 的包路径有没有被修改过吧，可能是包升级过了：
import werkzeug

[killva4624]

@killva4624 print(werkzeug.__file__)

[zxCoder]

@killva4624 唉 迷惑 werkzeug 从 2.0.1 降到 2.0.0 就好了。。。。

[killva4624]

感觉是 contextvars 这个包的问题
local.py:

```python
try:
from contextvars import ContextVar

if "gevent" in sys.modules or "eventlet" in sys.modules:
# Both use greenlet, so first check it has patched
# ContextVars, Greenlet <0.4.17 does not.
import greenlet

greenlet_patched = getattr(greenlet, "GREENLET_USE_CONTEXT_VARS", False)

if not greenlet_patched:
# If Gevent is used, check it has patched ContextVars,
# <20.5 does not.
try:
from gevent.monkey import is_object_patched
except ImportError:
# Gevent isn't used, but Greenlet is and hasn't patched
raise _CannotUseContextVar()
else:
if is_object_patched("threading", "local") and not is_object_patched(
"contextvars", "ContextVar"
):
raise _CannotUseContextVar()


except (ImportError, _CannotUseContextVar):

class ContextVar: # type: ignore
"""A fake ContextVar based on the previous greenlet/threading
ident function. Used on Python 3.6, eventlet, and old versions
of gevent.
"""

def __init__(self, _name: str) -> None:
self.storage: t.Dict[int, t.Dict[str, t.Any]] = {}

def get(self, default: t.Dict[str, t.Any]) -> t.Dict[str, t.Any]:
return self.storage.get(_get_ident(), default)

def set(self, value: t.Dict[str, t.Any]) -> None:
self.storage[_get_ident()] = value
```