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.

[gistrec]

Adding to the existing comment: this is caused by running the Flask development server in threaded mode and then entering pdbpp from inside a request handler.

Since Flask 1.0, the development server runs with threaded=True by default. That means each request is handled in a worker thread. Python only allows signal.signal() to be called from the main thread of the main interpreter, and pdbpp installs signal handlers when it starts. So when breakpoint() is hit inside the request thread, pdbpp fails with:

ValueError: signal only works in main thread of the main interpreter

For local interactive debugging, disable request threading:

if __name__ == "__main__":
    app.run(debug=True, threaded=False)

Or, if you start the app with the Flask CLI:

flask run --without-threads

With threading disabled, requests are handled sequentially on the main thread, so breakpoint() / pdbpp should work as expected.

The trade-off is that the development server can process only one request at a time, but that is usually fine while you are stopped in an interactive debugger anyway.

If you need to keep threaded behavior, use an IDE/debugger such as VS Code, PyCharm, or debugpy, since those are better suited for breakpoints in worker threads.

[kmille]

Hm. I'm currently having this issue again with

app.run(debug=True,
        threaded=False,
        host="127.0.0.1",
        port=5000)