Flask CLI shows “TypeError: exceptions must derive from BaseException”

When I run Flask from the terminal with flask run, if my code has an error I see “TypeError: exceptions must derive from BaseException” instead of the actual error. To debug it I need to open the browser and inspect the exc_info variable in the Werkzeug debugger, which is not convenient. Why is Flask doing this? How can I see the actual error in the terminal?

$ FLASK_APP=example flask run

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

JavaScript
​x
 
$ FLASK_APP=example flask run
​
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
​

Answer

This is a bug introduced in Flask 2.0 that will be fixed when Flask 2.0.2 is released. The CLI was incorrectly re-raising sys.exc_info() instead of only the exception object. Until then, you can turn off lazy loading by running with flask run --eager-loading.

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Answer