flask and pdbpp gives signal only works in main thread of the main interpreter

[kmille]

This is my code example:

from flask import Flask

app = Flask(__name__)


@app.route("/status")
def status():
    breakpoint()
    return {"status": "ok"}


if __name__ == '__main__':
    app.run()

Accessing /status, I get:

-> breakpoint()
127.0.0.1 - - [12/Jul/2025 08:32:24] "GET /status HTTP/1.1" 500 -
Traceback (most recent call last):
  File "/home/kmille/.cache/pypoetry/virtualenvs/zammad-pgp-autoimport-webhook-jd1kDdnL-py3.13/lib/python3.13/site-packages/flask/app.py", line 1536, in __call__
    return self.wsgi_app(environ, start_response)
           ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/kmille/.cache/pypoetry/virtualenvs/zammad-pgp-autoimport-webhook-jd1kDdnL-py3.13/lib/python3.13/site-packages/flask/app.py", line 1514, in wsgi_app
    response = self.handle_exception(e)
  File "/home/kmille/.cache/pypoetry/virtualenvs/zammad-pgp-autoimport-webhook-jd1kDdnL-py3.13/lib/python3.13/site-packages/flask/app.py", line 1511, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/kmille/.cache/pypoetry/virtualenvs/zammad-pgp-autoimport-webhook-jd1kDdnL-py3.13/lib/python3.13/site-packages/flask/app.py", line 919, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/kmille/.cache/pypoetry/virtualenvs/zammad-pgp-autoimport-webhook-jd1kDdnL-py3.13/lib/python3.13/site-packages/flask/app.py", line 917, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/kmille/.cache/pypoetry/virtualenvs/zammad-pgp-autoimport-webhook-jd1kDdnL-py3.13/lib/python3.13/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 "/home/kmille/projects/zammad-pgp-autoimport-webhook/src/zammad_pgp_autoimport_webhook/test_run.py", line 8, in status
    breakpoint()
    ~~~~~~~~~~^^
  File "/usr/lib/python3.13/bdb.py", line 116, in trace_dispatch
    return self.dispatch_opcode(frame, arg)
           ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
  File "/usr/lib/python3.13/bdb.py", line 215, in dispatch_opcode
    self.user_opcode(frame)
    ~~~~~~~~~~~~~~~~^^^^^^^
  File "/usr/lib/python3.13/pdb.py", line 448, in user_line
    self.interaction(frame, None)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/home/kmille/.cache/pypoetry/virtualenvs/zammad-pgp-autoimport-webhook-jd1kDdnL-py3.13/lib/python3.13/site-packages/pdbpp.py", line 431, in interaction
    return self._interaction(frame, tb)
           ~~~~~~~~~~~~~~~~~^^^^^^^^^^^
  File "/home/kmille/.cache/pypoetry/virtualenvs/zammad-pgp-autoimport-webhook-jd1kDdnL-py3.13/lib/python3.13/site-packages/pdbpp.py", line 465, in _interaction
    self._cmdloop()
    ~~~~~~~~~~~~~^^
  File "/usr/lib/python3.13/pdb.py", line 511, in _cmdloop
    self.cmdloop()
    ~~~~~~~~~~~~^^
  File "/usr/lib/python3.13/cmd.py", line 134, in cmdloop
    line = input(self.prompt)
  File "/usr/lib/python3.13/_pyrepl/readline.py", line 364, in input
    reader = self.get_reader()
  File "/usr/lib/python3.13/_pyrepl/readline.py", line 358, in get_reader
    console = Console(self.f_in, self.f_out, encoding=ENCODING)
  File "/usr/lib/python3.13/_pyrepl/unix_console.py", line 202, in __init__
    signal.signal(signal.SIGCONT, self._sigcont_handler)
    ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/signal.py", line 58, in signal
    handler = _signal.signal(_enum_to_int(signalnum), _enum_to_int(handler))
ValueError: signal only works in main thread of the main interpreter

Versions I use

poetry show -l | grep -e flask -e pdbpp                        
flask              3.1.1     3.1.1    A simple framework for building comple...
pdbpp              0.11.6    0.11.6   pdb++, a drop-in replacement for pdb

Using Python 3.13.5.

Thanks for your help! I'm wondering why I'm the only one having this... Couldn't find something in the web...

[yesminehe]

The breakpoint() call is meant for interactive debugging and isn’t suitable for a running Flask application, especially in a multi-threaded environment. Remove it from the /status route

[kmille]

Yea, but I want to debug at some point. And it worked in the past...

…

On July 18, 2025 11:49:15 PM GMT+02:00, Yasmine Hichri ***@***.***> wrote: The breakpoint() call is meant for interactive debugging and isn’t suitable for a running Flask application, especially in a multi-threaded environment. Remove it from the /status route -- Reply to this email directly or view it on GitHub: #5771 (comment) You are receiving this because you authored the thread. Message ID: ***@***.***>

[yesminehe]

Switch to using logging or write intermediate outputs temporarily.