Debugging a running python process by interrupting and providing an interactive prompt (Python recipe) by Brian McErlean

ActiveState Code (http://code.activestate.com/recipes/576515/)

try: import readline  # For readline input support
except: pass

import sys, os, traceback, signal, codeop, cStringIO, cPickle, tempfile

def pipename(pid):
    """Return name of pipe to use"""
    return os.path.join(tempfile.gettempdir(), 'debug-%d' % pid)

class NamedPipe(object):
    def __init__(self, name, end=0, mode=0666):
        """Open a pair of pipes, name.in and name.out for communication
        with another process.  One process should pass 1 for end, and the
        other 0.  Data is marshalled with pickle."""
        self.in_name, self.out_name = name +'.in',  name +'.out',
        try: os.mkfifo(self.in_name,mode)
        except OSError: pass
        try: os.mkfifo(self.out_name,mode)
        except OSError: pass
        
        # NOTE: The order the ends are opened in is important - both ends
        # of pipe 1 must be opened before the second pipe can be opened.
        if end:
            self.inp = open(self.out_name,'r')
            self.out = open(self.in_name,'w')
        else:
            self.out = open(self.out_name,'w')
            self.inp = open(self.in_name,'r')
        self._open = True

    def is_open(self):
        return not (self.inp.closed or self.out.closed)
        
    def put(self,msg):
        if self.is_open():
            data = cPickle.dumps(msg,1)
            self.out.write("%d\n" % len(data))
            self.out.write(data)
            self.out.flush()
        else:
            raise Exception("Pipe closed")
        
    def get(self):
        txt=self.inp.readline()
        if not txt: 
            self.inp.close()
        else:
            l = int(txt)
            data=self.inp.read(l)
            if len(data) < l: self.inp.close()
            return cPickle.loads(data)  # Convert back to python object.
            
    def close(self):
        self.inp.close()
        self.out.close()
        try: os.remove(self.in_name)
        except OSError: pass
        try: os.remove(self.out_name)
        except OSError: pass

    def __del__(self):
        self.close()
        
def remote_debug(sig,frame):
    """Handler to allow process to be remotely debugged."""
    def _raiseEx(ex):
        """Raise specified exception in the remote process"""
        _raiseEx.ex = ex
    _raiseEx.ex = None
    
    try:
        # Provide some useful functions.
        locs = {'_raiseEx' : _raiseEx}
        locs.update(frame.f_locals)  # Unless shadowed.
        globs = frame.f_globals
        
        pid = os.getpid()  # Use pipe name based on pid
        pipe = NamedPipe(pipename(pid))
    
        old_stdout, old_stderr = sys.stdout, sys.stderr
        txt = ''
        pipe.put("Interrupting process at following point:\n" + 
               ''.join(traceback.format_stack(frame)) + ">>> ")
        
        try:
            while pipe.is_open() and _raiseEx.ex is None:
                line = pipe.get()
                if line is None: continue # EOF
                txt += line
                try:
                    code = codeop.compile_command(txt)
                    if code:
                        sys.stdout = cStringIO.StringIO()
                        sys.stderr = sys.stdout
                        exec code in globs,locs
                        txt = ''
                        pipe.put(sys.stdout.getvalue() + '>>> ')
                    else:
                        pipe.put('... ')
                except:
                    txt='' # May be syntax err.
                    sys.stdout = cStringIO.StringIO()
                    sys.stderr = sys.stdout
                    traceback.print_exc()
                    pipe.put(sys.stdout.getvalue() + '>>> ')
        finally:
            sys.stdout = old_stdout # Restore redirected output.
            sys.stderr = old_stderr
            pipe.close()

    except Exception:  # Don't allow debug exceptions to propogate to real program.
        traceback.print_exc()
        
    if _raiseEx.ex is not None: raise _raiseEx.ex
    
def debug_process(pid):
    """Interrupt a running process and debug it."""
    os.kill(pid, signal.SIGUSR1)  # Signal process.
    pipe = NamedPipe(pipename(pid), 1)
    try:
        while pipe.is_open():
            txt=raw_input(pipe.get()) + '\n'
            pipe.put(txt)
    except EOFError:
        pass # Exit.
    pipe.close()

def listen():
    signal.signal(signal.SIGUSR1, remote_debug) # Register for remote debugging.

if __name__=='__main__':
    if len(sys.argv) != 2:
        print "Error: Must provide process id to debug"
    else:
        pid = int(sys.argv[1])
        debug_process(pid)