深入分析在Python模块顶层运行的代码引起的一个Bug

> import subprocess
  > subprocess.check_call("false")
  0

> subprocess.check_call("false")
  Traceback (most recent call last):
   File "", line 1, in 
   File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 542, in check_call
    raise CalledProcessError(retcode, cmd)
  subprocess.CalledProcessError: Command 'false' returned non-zero exit status 1

def wait(self):
  """Wait for child process to terminate. Returns returncode attribute."""
  while self.returncode is None:
   try:
    pid, sts = _eintr_retry_call(os.waitpid, self.pid, 0)
   except OSError as e:
    if e.errno != errno.ECHILD:
     raise
    # This happens if SIGCLD is set to be ignored or waiting
    # for child processes has otherwise been disabled for our
    # process. This child is dead, we can't get the status.
    pid = self.pid
    sts = 0
   # Check the pid and loop as waitpid has been known to return
   # 0 even without WNOHANG in odd situations. issue14396.
   if pid == self.pid:
    self._handle_exitstatus(sts)
  return self.returncode