threading ignores KeyboardInterrupt exception

I’m running this simple code:

import threading, time

class reqthread(threading.Thread):    
    def run(self):
        for i in range(0, 10):
            time.sleep(1)
            print('.')

try:
    thread = reqthread()
    thread.start()
except (KeyboardInterrupt, SystemExit):
    print('n! Received keyboard interrupt, quitting threads.n')

But when I run it, it prints

$ python prova.py
.
.
^C.
.
.
.
.
.
.
.
Exception KeyboardInterrupt in <module 'threading' from '/usr/lib/python2.6/threading.pyc'> ignored

In fact python thread ignore my Ctrl+C keyboard interrupt and doesn’t print Received Keyboard Interrupt. Why? What is wrong with this code?

Answers:

Thank you for visiting the Q&A section on Magenaut. Please note that all the answers may not help you solve the issue immediately. So please treat them as advisements. If you found the post helpful (or not), leave a comment & I’ll get back to you as soon as possible.

Method 1

Try

try:
  thread=reqthread()
  thread.daemon=True
  thread.start()
  while True: time.sleep(100)
except (KeyboardInterrupt, SystemExit):
  print 'n! Received keyboard interrupt, quitting threads.n'

Without the call to time.sleep, the main process is jumping out of the try...except block too early, so the KeyboardInterrupt is not caught. My first thought was to use thread.join, but that seems to block the main process (ignoring KeyboardInterrupt) until the thread is finished.

thread.daemon=True causes the thread to terminate when the main process ends.

Method 2

To summarize the changes recommended in the comments, the following works well for me:

try:
  thread = reqthread()
  thread.start()
  while thread.isAlive(): 
    thread.join(1)  # not sure if there is an appreciable cost to this.
except (KeyboardInterrupt, SystemExit):
  print 'n! Received keyboard interrupt, quitting threads.n'
  sys.exit()

Method 3

Slight modification of ubuntu’s solution.

Removing tread.daemon = True as suggested by Eric and replacing the sleeping loop by signal.pause():

import signal
try:
  thread=reqthread()
  thread.start()
  signal.pause() # instead of: while True: time.sleep(100)
except (KeyboardInterrupt, SystemExit):
  print 'n! Received keyboard interrupt, quitting threads.n'

Method 4

My (hacky) solution is to monkey-patch Thread.join() like this:

def initThreadJoinHack():
  import threading, thread
  mainThread = threading.currentThread()
  assert isinstance(mainThread, threading._MainThread)
  mainThreadId = thread.get_ident()
  join_orig = threading.Thread.join
  def join_hacked(threadObj, timeout=None):
    """
    :type threadObj: threading.Thread
    :type timeout: float|None
    """
    if timeout is None and thread.get_ident() == mainThreadId:
      # This is a HACK for Thread.join() if we are in the main thread.
      # In that case, a Thread.join(timeout=None) would hang and even not respond to signals
      # because signals will get delivered to other threads and Python would forward
      # them for delayed handling to the main thread which hangs.
      # See CPython signalmodule.c.
      # Currently the best solution I can think of:
      while threadObj.isAlive():
        join_orig(threadObj, timeout=0.1)
    else:
      # In all other cases, we can use the original.
      join_orig(threadObj, timeout=timeout)
  threading.Thread.join = join_hacked

Method 5

Putting the try ... except in each thread and also a signal.pause() in true main() works for me.

Watch out for import lock though. I am guessing this is why Python doesn’t solve ctrl-C by default.

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0