Python piping output between two subprocesses

I'm working on some code that will DD a block device over SSH, and I'm wanting to do this with subprocess so that I can monitor the status of DD during the transfer (killing the dd process with SIGUSR1 to get its current state, and reading that using selects).

The com开发者_Go百科mand that I'm trying to implement would be something like this:

dd if=/dev/sda | ssh root@example.com 'dd of=/dev/sda'

The current method I tried was:

dd_process = subprocess.Popen(['dd','if=/dev/sda'],0,None,None,subprocess.PIPE, subprocess.PIPE)  
ssh_process = subprocess.Popen(['ssh','root@example.com','dd of=/dev/sda'],0,None,dd_process.stdout)

However when I run this, the SSH process becomes defunct after 10-40 seconds.

Am I being completely obtuse here, or is there no way to pipe between subprocesses like this?

Edit: Turns out my real code didn't have the hostname in it. This is the correct way to do things.

from subprocess import Popen, PIPE
dd_process = Popen(['dd', 'if=/dev/sda'], stdout=PIPE)
ssh_process = Popen(['ssh', 'root@example.com', 'dd','of=/dev/sda'],stdin=dd_process.stdout, stdout=PIPE)
dd_process.stdout.close() # enable write error in dd if ssh dies
out, err = ssh_process.communicate()

This is way to PIPE the first process output to the second. (notice stdin in the ssh_process)

The sh Python library makes it easy to call OS commands and pipe them.

From the documentation:

for line in tr(tail("-f", "test.log", _piped=True), "[:upper:]", "[:lower:]", _iter=True):
    print(line)