Is there a way to attach a debugger to a multi-threaded Python process?

Question

I\'m trying to debug a deadlock in a multi-threaded Python application after it has locked up. Is there a way to attach a debugger to inspect the state of the process?

Answer 1

pdbinject allows you to inject pdb into an already running python process.

The pdbinject executable only works under python2, but can inject into python3 just fine too.

Answer 2

This can be used as a dead simple "remote" debugger:

import sys
import socket
import pdb

def remote_trace():
    server = socket.socket()
    server.bind(('0.0.0.0', 12345))
    server.listen()
    client, _= server.accept()
    stream = client.makefile('rw')
    sys.stdin = sys.stdout = sys.stderr = stream
    pdb.set_trace()

remote_trace()

# Execute in the shell: `telnet 127.0.0.1 12345`

On Windows it's easier to use Netcat instead of Telnet (which will also work on linux).

Answer 3

Yeah, gdb is good for lower level debugging.

You can change threads with the thread command.

e.g

(gdb) thr 2
[Switching to thread 2 (process 6159 thread 0x3f1b)]
(gdb) backtrace
....

You could also check out Python specific debuggers like Winpdb, or pydb. Both platform independent.