Python中执行外部命令并捕获双向输出

import subprocess
# print ’popen3:’
def external_cmd(cmd, msg_in=''):   
    try:       
        proc = subprocess.Popen(cmd, shell=True, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE,)        
        stdout_value, stderr_value = proc.communicate(msg_in) 
        return stdout_value, stderr_value    
    except ValueError, err: 
        # log("IOError: %s" % err) 
        return None, None

if __name__ == '__main__':    
    stdout_val, stderr_val = external_cmd('ls -l')    
    print 'Standard Output: %s' % stdout_val    
    print 'Standard Error: %s' % stderr_val