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How to print stdout before writing stdin using subprocess module in Python?

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I am writing a script in which in the external system command may sometimes require user input. I am not able to handle that properly. I have tried using os.popen4 and subprocess module but could not achieve the desired behavior.

Below mentioned example would show this problem using "cp" command. ("cp" command is used to show this problem, i am calling some different exe which may similarly prompt for user response in some scenarios). In this example there are two files present on disk and when user tries to copy file1 to file2, an conformer message comes up. 

proc = subprocess.Popen("cp -i a.txt b.txt", shell=True, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.STDOUT,) stdout_val, stderr_val = proc.communicate() print stdout_val b.txt?

proc.communicate("y") 

Now in this example if i read only stdout/stderr and prints it, later on if i try to write "y" or "n" based on user's input, i got an error that channel is closed. Can some one please help me on achieving this behavior in python such that i can print stdout first, then should take user input and write stdin later on.
 

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How to print stdout before writing stdin using subprocess module in Python? Sarbjit singh <sarbjit1987@gmail.com> - 2012-07-23 06:01 -0700
  Re: How to print stdout before writing stdin using subprocess module in Python? Peter Otten <__peter__@web.de> - 2012-07-23 15:38 +0200
  Re: How to print stdout before writing stdin using subprocess module in Python? Nobody <nobody@nowhere.com> - 2012-07-23 19:29 +0100

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