Referencing other modules in atexit

I have a function that is responsible for killing a child process when the program ends:

class MySingleton:
    def __init__(self):开发者_如何学运维
        import atexit
        atexit.register(self.stop)

    def stop(self):
        os.kill(self.sel_server_pid, signal.SIGTERM)

However I get an error message when this function is called:

Traceback (most recent call last):
File "/usr/lib/python2.5/atexit.py", line 24, in _run_exitfuncs
   func(*targs, **kargs)
File "/home/commando/Development/Diploma/streaminatr/stream/selenium_tests.py", line 66, in stop
   os.kill(self.sel_server_pid, signal.SIGTERM)
AttributeError: 'NoneType' object has no attribute 'kill'

Looks like the os and signal modules get unloaded before atexit is called. Re-importing them solves the problem, but this behaviour seems weird to me - these modules are imported before I register my handler, so why are they unloaded before my own exit handler runs?

There are no strong guarantees about the order in which things are destroyed at program termination time, so it's best to ensure atexit-registered functions are self contained. E.g., in your case:

class MySingleton:
    def __init__(self):
        import atexit
        atexit.register(self.stop)
        self._dokill = os.kill
        self._thesig = signal.SIGTERM

    def stop(self):
        self._dokill(self.sel_server_pid, self._thesig)

This is preferable to re-importing modules (which could conceivably cause slowdown of program termination and even unending loops, though that risk is lesser for "system-supplied" modules such as os).