How to attach debugger to a python subproccess?

Question

I need to debug a child process spawned by multiprocessing.Process(). The pdb degugger seems to be unaware of forking and unable to attach to alrea

Answer 1

Building upon @memplex idea, I had to modify it to get it to work with joblib by setting the sys.stdin in the constructor as well as passing it directly along via joblib.

import os
import pdb
import signal
import sys
import joblib

_original_stdin_fd = None

class ForkablePdb(pdb.Pdb):
    _original_stdin = None
    _original_pid = os.getpid()

    def __init__(self):
        pdb.Pdb.__init__(self)
        if self._original_pid != os.getpid():
            if _original_stdin_fd is None:
                raise Exception("Must set ForkablePdb._original_stdin_fd to stdin fileno")

            self.current_stdin = sys.stdin
            if not self._original_stdin:
                self._original_stdin = os.fdopen(_original_stdin_fd)
            sys.stdin = self._original_stdin

    def _cmdloop(self):
        try:
            self.cmdloop()
        finally:
            sys.stdin = self.current_stdin

def handle_pdb(sig, frame):
    ForkablePdb().set_trace(frame)

def test(i, fileno):
    global _original_stdin_fd
    _original_stdin_fd = fileno
    while True:
        pass    

if __name__ == '__main__':
    print "PID: %d" % os.getpid()
    signal.signal(signal.SIGUSR2, handle_pdb)
    ForkablePdb().set_trace()
    fileno = sys.stdin.fileno()
    joblib.Parallel(n_jobs=2)(joblib.delayed(test)(i, fileno) for i in range(10))