首先，在python的线程中，任何在run函数中调用的代码，都是运行在新线程中。

其他的实例方法，全部运行在主线程中。

上代码：

#!/usr/bin/python
# -- coding: utf-8 --

import threading
import signal
import time

# 注意：run函数中执行的代码都是在新线程中
# 而hander方法在主线程中，可以查看它们的thread id

def thread_sig():
    # 在子线程中发送信号
    signal.alarm(3)

class ihander(threading.Thread):
    def __init__(self):
        super(ihander, self).__init__()
        print threading.currentThread(), " in __init__"
        signal.signal(signal.SIGALRM, self.handler)
    
    def run(self):
        print threading.currentThread(), " in run"
        time.sleep(10)
    
    def handler(self, signum, frame):
        print threading.currentThread(), " in handler"
        print 'signal: ', signum

h = ihander()
h.start()

t = threading.Thread(target=thread_sig, args=())
t.start()

执行结果：

<_MainThread(MainThread, started 139779179116288)>  in __init__
<ihander(Thread-1, started 139779145524992)>  in run
<_MainThread(MainThread, stopped 139779179116288)>  in handler
signal:  14

我们如果把signal.signal放在其他线程中，就会出错：

def thread_sig():
    # 在子线程中发送信号
    signal.alarm(3)
    signal.signal(signal.SIGALRM, h.handler)

class ihander(threading.Thread):
    def __init__(self):
        super(ihander, self).__init__()
        print threading.currentThread(), " in __init__"
    
    def run(self):
        print threading.currentThread(), " in run"
        time.sleep(10)
    
    def handler(self, signum, frame):
        print threading.currentThread(), " in handler"
        print 'signal: ', signum

执行结果是：

Exception in thread Thread-2:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 551, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 504, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/data/tornado_analyst/test_signal.py", line 14, in thread_sig
    signal.signal(signal.SIGALRM, h.handler)
ValueError: signal only works in main thread

python报告说，signal只能工作在主线程中。

下面是一些关于python中信号的注意点：

/*
   NOTES ON THE INTERACTION BETWEEN SIGNALS AND THREADS

   When threads are supported, we want the following semantics:

   - only the main thread can set a signal handler
   - any thread can get a signal handler
   - signals are only delivered to the main thread

   I.e. we don't support "synchronous signals" like SIGFPE (catching
   this doesn't make much sense in Python anyway) nor do we support
   signals as a means of inter-thread communication, since not all
   thread implementations support that (at least our thread library
   doesn't).

   We still have the problem that in some implementations signals
   generated by the keyboard (e.g. SIGINT) are delivered to all
   threads (e.g. SGI), while in others (e.g. Solaris) such signals are
   delivered to one random thread (an intermediate possibility would
   be to deliver it to the main thread -- POSIX?).  For now, we have
   a working implementation that works in all three cases -- the
   handler ignores signals if getpid() isn't the same as in the main
   thread.  XXX This is a hack.

   GNU pth is a user-space threading library, and as such, all threads
   run within the same process. In this case, if the currently running
   thread is not the main_thread, send the signal to the main_thread.
*/