Exceptions are sometimes replaced with "TypeError: exceptions must derive from BaseException"

[sbronson]

# a.py
def create_app(): raise RuntimeError()

$ FLASK_APP=a.py flask run --lazy-loading
$ curl http://127.0.0.1:5000
[...]
Traceback (most recent call last):
  File "[...]/lib/python3.9/site-packages/flask/cli.py", line 356, in __call__
    self._flush_bg_loading_exception()
  File "[...]/lib/python3.9/site-packages/flask/cli.py", line 344, in _flush_bg_loading_exception
    raise exc_info
TypeError: exceptions must derive from BaseException

I expected something about a RuntimeError. raise exc_info[1] here worked for me

flask/src/flask/cli.py

Line 342 in 7161776

raise exc_info

Environment:

• Python version: 3.9.5
• Flask version: 2.0.1

[cygniv404]

for me it was because i had the same function names for the same endpoint

[thnee]

Yes thank you for reporting this, glad I am not the only one, thought I was going crazy!

This happens for any exception from anywhere in the project, but only sometimes, seemingly at random. It gets replaced with this TypeError instead of the expected exception.

I have only observed this while running with the flask run dev server, not in production.

It seems to happen right after server autoreload on code update. It seems to happen on the first load of the page after server reload. When reloading the page again, the problem always goes away, and the expected exception is raised.