RuntimeError: The session is unavailable because no secret key was set. Set the secret_key on the a

诸神缄默不语-个人技术博文与视频目录

这是在运行flask代码的过程中，在修改flask.session中属性的值时报的错。完整报错信息如下：
（这个文件路径之所以长这样是因为在docker里，总之这不重要）

Traceback (most recent call last):
  File "/usr/local/lib/python3.10/site-packages/flask/app.py", line 1536, in __call__
    return self.wsgi_app(environ, start_response)
  File "/usr/local/lib/python3.10/site-packages/flask/app.py", line 1514, in wsgi_app
    response = self.handle_exception(e)
  File "/usr/local/lib/python3.10/site-packages/flask/app.py", line 1511, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python3.10/site-packages/flask/app.py", line 919, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python3.10/site-packages/flask/app.py", line 917, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python3.10/site-packages/flask/app.py", line 902, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)  # type: ignore[no-any-return]
  File "/app/flaskapp.py", line 65, in login
    session["k"] = v
  File "/usr/local/lib/python3.10/site-packages/flask/sessions.py", line 104, in _fail
    raise RuntimeError(
RuntimeError: The session is unavailable because no secret key was set.  Set the secret_key on the application to something unique and secret.

这个报错信息就是因为没有设置Flask.secret_key，解决方案是首先生成一个随机初始化的字节对象（新版本现在用字符串对象也可以了）作为秘钥，如b'_5#y2L"F4Q8z\n\xec]/'（但是不要用这个，意思是让你自己生成一个）。
可以用代码生成：

import secrets
print(secrets.token_hex())

然后在代码最开头app = Flask(__name__)的下一行写app.secret_key = app_secret_key
就OK了。