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

An idea I had was to create "dummy" classes to fake the implementation of the methods you are using from multiprocessing:

from multiprocessing import Pool


class DummyPool():
    @staticmethod
    def apply_async(func, args, kwds):
        return DummyApplyResult(func(*args, **kwds))

    def close(self): pass
    def join(self): pass


class DummyApplyResult():
    def __init__(self, result):
        self.result = result

    def get(self):
        return self.result


def foo(a, b, switch):
    # set trace when DummyPool is used
    # import ipdb; ipdb.set_trace()
    if switch:
        return b - a
    else:
        return a - b


if __name__ == '__main__':
    xml = etree.parse('C:/Users/anmendoza/Downloads/jim - 8.1/running-config.xml')
    pool = DummyPool()  # switch between Pool() and DummyPool() here
    results = []
    results.append(pool.apply_async(foo, args=(1, 100), kwds={'switch': True}))
    pool.close()
    pool.join()
    results[0].get()

Answer 2

This is an elaboration of Romuald's answer which restores the original stdin using its file descriptor. This keeps readline working inside the debugger. Besides, pdb special management of KeyboardInterrupt is disabled, in order it not to interfere with multiprocessing sigint handler.

class ForkablePdb(pdb.Pdb):

    _original_stdin_fd = sys.stdin.fileno()
    _original_stdin = None

    def __init__(self):
        pdb.Pdb.__init__(self, nosigint=True)

    def _cmdloop(self):
        current_stdin = sys.stdin
        try:
            if not self._original_stdin:
                self._original_stdin = os.fdopen(self._original_stdin_fd)
            sys.stdin = self._original_stdin
            self.cmdloop()
        finally:
            sys.stdin = current_stdin

Answer 3

Here is the version of the ForkedPdb(Romuald's Solution) which will work for Windows and *nix based systems.

import sys
import pdb
import win32console


class MyHandle():
    def __init__(self):
        self.screenBuffer = win32console.GetStdHandle(win32console.STD_INPUT_HANDLE)
    
    def readline(self):
        return self.screenBuffer.ReadConsole(1000)

class ForkedPdb(pdb.Pdb):
    def interaction(self, *args, **kwargs):
        _stdin = sys.stdin
        try:
            if sys.platform == "win32":
                sys.stdin = MyHandle()
            else:
                sys.stdin = open('/dev/stdin')
            pdb.Pdb.interaction(self, *args, **kwargs)
        finally:
            sys.stdin = _stdin

Answer 4

I've been searching for a simple to solution for this problem and came up with this:

import sys
import pdb

class ForkedPdb(pdb.Pdb):
    """A Pdb subclass that may be used
    from a forked multiprocessing child

    """
    def interaction(self, *args, **kwargs):
        _stdin = sys.stdin
        try:
            sys.stdin = open('/dev/stdin')
            pdb.Pdb.interaction(self, *args, **kwargs)
        finally:
            sys.stdin = _stdin

Use it the same way you might use the classic Pdb:

ForkedPdb().set_trace()

Answer 5

Winpdb is pretty much the definition of a smarter Python debugger. It explicitly supports going down a fork, not sure it works nicely with multiprocessing.Process() but it's worth a try.

For a list of candidates to check for support of your use case, see the list of Python Debuggers in the wiki.

Answer 6

If you are on a supported platform, try DTrace. Most of the BSD / Solaris / OS X family support DTrace.

Here is an intro by the author. You can use Dtrace to debug just about anything.

Here is a SO post on learning DTrace.