Use Popen as Context Manager Preferably

It is preferred to use Popen() in Python subprocess lib as context manager, to properly close all running subprocesses. Otherwise subprocesses may still be running when the main process exits. According to the documentation,

on exit, standard file descriptors are closed, and the process is waited for.

For example, to process on output of piped commands and be able to pre-emptively terminate the pipeline while properly closing all subprocesses:

from subprocess import Popen, PIPE

with Popen(<cmd1>, stdout=PIPE, text=True) as p1:
    with Popen(<cmd2>, stdin=p1.stdout, stdout=PIPE, text=True) as p2:
        with Popen(<cmd3>, stdin=p2.stdout, stdout=PIPE, text=True) as p3:
            # process on p3.stdout

# outside the nested contexts all subprocesses have been terminated