signal handling changes in pyrepl break pdbpp on python>=3.13.4

[distrame]

I'm getting an error ValueError: signal only works in main thread of the main interpreter when using breakpoint() inside fastapi route handler when pdbpp is installed.

To reproduce:

uv init -p 3.13.5 --app test-pdbpp
cd test-pdbpp/
uv add fastapi[standard] pdbpp
# replace main.py contents
uv run fastapi dev
curl 127.0.0.1:8000

The main.py:

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
def read_root():
    breakpoint()
    return {"Hello": "World"}

Full traceback

[3] > /app/test-pdbpp/main.py(8)read_root()
-> breakpoint()
      INFO   127.0.0.1:47460 - "GET / HTTP/1.1" 500
     ERROR   Exception in ASGI application
Traceback (most recent call last):
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/uvicorn/protocols/http/httptools_impl.py", line 409, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        self.scope, self.receive, self.send
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/uvicorn/middleware/proxy_headers.py", line 60, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/fastapi/applications.py", line 1054, in __call__
    await super().__call__(scope, receive, send)
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/starlette/applications.py", line 113, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/starlette/middleware/errors.py", line 186, in __call__
    raise exc
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/starlette/middleware/errors.py", line 164, in __call__
    await self.app(scope, receive, _send)
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/starlette/middleware/exceptions.py", line 63, in __call__
    await wrap_app_handling_exceptions(self.app, conn)(scope, receive, send)
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
    raise exc
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/starlette/_exception_handler.py", line 42, in wrapped_app
    await app(scope, receive, sender)
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/starlette/routing.py", line 716, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/starlette/routing.py", line 736, in app
    await route.handle(scope, receive, send)
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/starlette/routing.py", line 290, in handle
    await self.app(scope, receive, send)
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/starlette/routing.py", line 78, in app
    await wrap_app_handling_exceptions(app, request)(scope, receive, send)
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
    raise exc
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/starlette/_exception_handler.py", line 42, in wrapped_app
    await app(scope, receive, sender)
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/starlette/routing.py", line 75, in app
    response = await f(request)
               ^^^^^^^^^^^^^^^^
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/fastapi/routing.py", line 302, in app
    raw_response = await run_endpoint_function(
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ...<3 lines>...
    )
    ^
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/fastapi/routing.py", line 215, in run_endpoint_function
    return await run_in_threadpool(dependant.call, **values)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/starlette/concurrency.py", line 38, in run_in_threadpool
    return await anyio.to_thread.run_sync(func)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/anyio/to_thread.py", line 56, in run_sync
    return await get_async_backend().run_sync_in_worker_thread(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        func, args, abandon_on_cancel=abandon_on_cancel, limiter=limiter
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/anyio/_backends/_asyncio.py", line 2476, in run_sync_in_worker_thread
    return await future
           ^^^^^^^^^^^^
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/anyio/_backends/_asyncio.py", line 967, in run
    result = context.run(func, *args)
  File "/app/test-pdbpp/main.py", line 8, in read_root
    breakpoint()
    ~~~~~~~~~~^^
  File "/usr/local/lib/python3.13/bdb.py", line 116, in trace_dispatch
    return self.dispatch_opcode(frame, arg)
           ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/bdb.py", line 215, in dispatch_opcode
    self.user_opcode(frame)
    ~~~~~~~~~~~~~~~~^^^^^^^
  File "/usr/local/lib/python3.13/pdb.py", line 448, in user_line
    self.interaction(frame, None)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/pdbpp.py", line 449, in interaction
    return self._interaction(frame, tb)
           ~~~~~~~~~~~~~~~~~^^^^^^^^^^^
  File "/app/test-pdbpp/.venv/lib/python3.13/site-packages/pdbpp.py", line 483, in _interaction
    self._cmdloop()
    ~~~~~~~~~~~~~^^
  File "/usr/local/lib/python3.13/pdb.py", line 511, in _cmdloop
    self.cmdloop()
    ~~~~~~~~~~~~^^
  File "/usr/local/lib/python3.13/cmd.py", line 134, in cmdloop
    line = input(self.prompt)
  File "/usr/local/lib/python3.13/_pyrepl/readline.py", line 364, in input
    reader = self.get_reader()
  File "/usr/local/lib/python3.13/_pyrepl/readline.py", line 358, in get_reader
    console = Console(self.f_in, self.f_out, encoding=ENCODING)
  File "/usr/local/lib/python3.13/_pyrepl/unix_console.py", line 202, in __init__
    signal.signal(signal.SIGCONT, self._sigcont_handler)
    ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/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

[saucoide]

reproduced in macos with python 3.13.5 & 3.13.4, older versions all seem to work as normal, didnt try 3.14 bc pydantic-core still doesnt build for it

[salty-horse]

This is the Python change that broke it: gh-132017: Handle SIGCONT in pyrepl

[salty-horse]

The title should be changed, as this is not about FastAPI.

Any ideas how to fix this?

[bretello]

@salty-horse you could try disabling pyrepl in the pdbpp config, see here: pdbpp#66 (comment)

[salty-horse]

@bretello Why? Unless I missed something in my testing, it works fine with 0.12.0rc1.

EDIT: I was wrong.

[davidwik]

@bretello Thanks, that solved my issues disabling the pyrepl.