Question

I’m using the following to execute a process and hide its output from Python. It’s in a loop though, and I need a way to block until the sub process has terminated before moving to the next iteration.

subprocess.Popen(["scanx", "--udp", host], stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE) 

Answer 1

Use subprocess.call(). From the docs:

subprocess.call(*popenargs, **kwargs)
Run command with arguments. Wait for command to complete, then
return the returncode attribute.
The arguments are the same as for the
Popen constructor.

Edit:

subprocess.call() uses wait(), and wait() is vulnerable to deadlocks (as Tommy Herbert pointed out). From the docs:

Warning: This will deadlock if the
child process generates enough output
to a stdout or stderr pipe such that
it blocks waiting for the OS pipe
buffer to accept more data. Use
communicate() to avoid that.

So if your command generates a lot of output, use communicate() instead:

p = subprocess.Popen(
    ["scanx", "--udp", host],
    stdin=subprocess.PIPE,
    stdout=subprocess.PIPE,
    stderr=subprocess.PIPE)
out, err = p.communicate()