popen4

[billie]

Hi all. I'm trying to execute system commands and capture the output by 
using popen4:

stdout example:
>>>exec_cmd = popen2.popen4("echo hello!")
>>>output = exec_cmd[0].read()
hello

stderr example:
>>>exec_cmd = popen2.popen4("echobv hello!")
>>>output = exec_cmd[0].read()
Unrecognized command

The problem occurs when I try to execute interactive commands like ftp, 
python intepreter etc...
In this case the program crashes without even giving an error.
Suggestions?

Regards