Popen.communicate完成后将设置returncode属性（*）。这是相关的文档部分：

Popen.returncode 
  The child return code, set by poll() and wait() (and indirectly by communicate()). 
  A None value indicates that the process hasn’t terminated yet.

  A negative value -N indicates that the child was terminated by signal N (Unix only).

所以您可以做（我没有测试过，但是应该可以）：

import subprocess as sp
child = sp.Popen(openRTSP + opts.split(), stdout=sp.PIPE)
streamdata = child.communicate()[0]
rc = child.returncode

（*）发生这种情况的原因是它的实现方式：设置线程以读取子流后，它仅调用wait。