[不懂就问] python subprocess 模块遇到一个疑问

from multiprocessing import Process
import os

def subpro():
    print 'there is a sub process which pid is %s' %os.getpid()

if __name__ == '__main__':
    p=Process(target=subpro, name='process_start_test')
    p.start()
    print 'there is a parent process which pid id %s' %os.getpid()

那么从官方文档看这个模块的说明，在实例化 Process 这个类的时候如果不传入参数 target 的话，执行 start 方法会调用 Process 类里的 run 方法

但是小弟不才，琢磨了半天 subprocess 源码，愣是看不出 start 方法在没有 target 属性是如何调用 run 方法的，以下为节选 subprocess 代码

def run(self):
        '''
        Method to be run in sub-process; can be overridden in sub-class
        '''
        if self._target:
            self._target(*self._args, **self._kwargs)

    def start(self):
        '''
        Start child process
        '''
        assert self._popen is None, 'cannot start a process twice'
        assert self._parent_pid == os.getpid(), \
               'can only start a process object created by current process'
        assert not _current_process._daemonic, \
               'daemonic processes are not allowed to have children'
        _cleanup()
        if self._Popen is not None:
            Popen = self._Popen
        else:
            from .forking import Popen
        self._popen = Popen(self)
        _current_process._children.add(self)