ssh via python subprocess.Popen: terminate if pass

I'm using a python script to manage ssh fingerprint problems after a workstation(s) is reimaged.

I attempt to connect with ssh, and if I get a any warnings I deal with them.

However, if there are no errors, then I am asked for a password to connect. At this point I want to terminate the process. However, the script hangs on the password request.

Here's the method:

def ssh_fingerprint_changed(node):
    """
    Checks if a node's ssh fingerprint has changed or an old key is found, which can occur when a node is reimaged.
    It does this by attempting to connect via ssh and inspecting stdout for an error message.
    :param node: the ip or hostname of the node
    :return: True if the node's fingerprint doesn't match the client's records. Else False.
    """
    changed = False
    cmd = ["ssh", "-q", ADMIN_USER + "@" + node, "exit"]
    proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, stdin=subprocess.PIPE, universal_newlines=True)
    print("Checking for fingerprint changes")

    for line in proc.stdout:  # loop on lines
        print("in for loop") # NEVER REACHES HERE IF NO ERRORS, WAITING FOR PASSWORD
        if b"Offending key" in line:
            print("Offending key found.")
            proc.stdin.write(b"no\n")   # don't connect
            changed = True
        elif b"REMOTE HOST IDENTIFICATION HAS CHANGED!" in line:
            print("REMOTE HOST IDENTIFICATION HAS CHANGED!")
            changed = True

    print(changed) # NEVER REACHES HERE IF NO ERRORS, WAITING FOR PASSWORD
    if not changed:  # then everything's good, but it will be waiting for a password to connect
        print("Good to go, terminating ssh test.")
        rc = proc.terminate()
    else:
        rc = proc.wait()

    return changed

If I run this from the terminal ./my_python_script.py, I have the problems. Oddly, if I run in PyCharm, it doesn't hang on the password request and terminates shh, continuing with the script as expected.

The easy answer is simply to tell ssh that you don't want to support password authentication at all; you'll still get the messages you want if the host key is changed, but you won't ever have the process hanging waiting for a password to be entered.

cmd = ['ssh',
       '-o', 'PasswordAuthentication no',  ### <-- THIS LINE HERE
       '-o', 'StrictHostKeyChecking yes',  ### also, never modify known_hosts
       '-q',
       '%s@%s' % (ADMIN_USER, + node),
       'exit']

If you did not want to process other prompts, I would suggest setting stdin=subprocess.DEVNULL (in Python 3) or passing the -n argument to ssh to prevent stdin from being passed to the process at all.