如何从subprocess.Popen获取退出代码？

为什么我没有获得退出代码？

import os
import sys
import subprocess

cmd = ['echo','hello']
p = subprocess.Popen(cmd,stdout=subprocess.PIPE,stderr=subprocess.STDOUT)
try:
    # Filter stdout
    for line in iter(p.stdout.readline,''):
        sys.stdout.flush()
        # Print status
        print(">>> " + line.rstrip())
        sys.stdout.flush()
except:
    sys.stdout.flush()

print 'RETURN CODE',p.returncode

请注意：我之所以单独阅读每一行是因为我想实时过滤其他长期运行的进程的输出,并根据某些字符串暂停它们.

我使用的是Python 2.7.5(CentOS 7 64位).

解

感谢@skyking发布的答案,我现在可以成功捕获这样的退出代码,使用Popen.poll()(Popen.wait()死锁我的进程)：

import os
import sys
import subprocess
import time

cmd = ['echo',''):
        sys.stdout.flush()
        # Print status
        print(">>> " + line.rstrip())
        sys.stdout.flush()
except:
    sys.stdout.flush()

# Wait until process terminates (without using p.wait())
while p.poll() is None:
    # Process hasn't exited yet,let's wait some
    time.sleep(0.5)

# Get return code from process
return_code = p.returncode

print 'RETURN CODE',return_code

# Exit with return code from process
sys.exit(return_code)