The sample is similar except for how stdout is handled there's no more calls toĬommunicate instead, proc.wait just waits for the child to exit (after TimeoutExpired : print ( 'subprocess did not terminate in time' ) t. wait ( timeout = 0.2 ) print ( '= subprocess exited with rc =', proc. urlopen ( ' ) assert b 'Directory listing' in resp. sleep ( 0.2 ) for i in range ( 4 ): resp = urllib. Thread ( target = output_reader, args = ( proc ,)) t. decode ( 'utf-8' )), end = '' ) def main (): proc = subprocess.

If you run this script, you'll see the output:ĭef output_reader ( proc ): for line in iter ( proc. A more sophisticated technique couldīe to send the child a SIGKILL (with proc.kill) if it didn't exit due to

If the child does not exit for some reason. communicate has a veryĬonvenient timeout argument starting with Python 3.3, letting us know Then, municate waitsįor the child to exit and captures all of its stdout. Sends the child process a SIGTERM signal. The meat of the sample happens in the finally block. Note the -u passed to Python on invocation: this is critical to avoid stdoutīuffering and seeing as much of stdout as possible when the process is killed.īuffering is a serious issue when interacting with child processes, and we'll Returns immediately and the child process runs in the background). Popen API to launch the process asynchronously (meaning that Popen Serving contents from the directory it was launched in. The child process is an HTTP server using Python's own rver module, TimeoutExpired : print ( 'subprocess did not terminate in time' ) communicate ( timeout = 0.2 ) print ( '= subprocess exited with rc =', proc. This is the use-case this post addresses.ĭef main (): proc = subprocess. Some sort of 'quit' command), this is doable. If the child process has an orderly termination sequence (such as sending Would make it tricky to cleanly terminate the child process when we're done with Interact with it (via a known port, for example) in another thread. Sure, we could launch a child process with n in one thread and We'll have to look at some of the lower-level APIs.

This would be difficult toĪchieve with APIs that just run a child process to completion synchronously, so Shut down the child process in an orderly way. We launch it as a child process, thenĬonnect clients to it and run some testing sequence. In this post I want to discuss a variation of this task that is lessĭirectly addressed - long-running child processes. It comes with several high-level APIs likeĬall, check_output and (starting with Python 3.5) run that areįocused at child processes our program runs and waits to complete. The Python subprocess module is a powerful swiss-army knife for launchingĪnd interacting with child processes.

Python launch subprocess in background