PYTHON SUBPROCESS GET OUTPUT FROM STDIN CODE

Here is to demonstrate what we have to do if the other options are unavailable.It might be better to use communicate: from subprocess import Popen, PIPE, STDOUT It would be better to send the child process some sort of 'exit'Ĭommand ( quit() in the case of the Python interpreter) the terminate In the end of the interaction, we close the child's stdin (this is optionalīut useful for some kinds of child processes) call terminate and then Since the child is still waiting for input to complete (step 1), our step 2 may We then invoke readline to wait for the reply.

Some reason (lack of newline, buffering etc.) We send data to the child's stdin, but it doesn't get the complete input for.We have to be very careful to avoid the following situation: Use readline to get input from the line interpreter.Always flush the stream after placing data into it, since it may be buffered.When sending input to a line interpreter, don't forget to send the actual.

PYTHON SUBPROCESS GET OUTPUT FROM STDIN CODE

Let me reiterate what the comment in this code sample is saying: PIPE ) # To avoid deadlocks: careful to: add to output, flush output, use # readline() rather than read() proc. The reader thread becomes:ĭef main (): proc = subprocess. With it (such as look for expected patterns), this is easy to organize with If we don't want to just print the captured stdout, but rather do something On that readline as soon as the child terminates, the readline call Will! As long as the child process is running, the thread will dutifully block Thus, while it may seem like the reader thread might never terminate - it always Only happens when proc.stdout is closed, which occurs when the child exits. (), until this call returns an empty bytestring. The iter(, b'') snippet is continously calling Run this sample, you'll notice that the child's stdout is reported in real-time, Looping as long as new lines are available and printing them immediately. A thread polls the child's stdout attribute, 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 subprocess get output from stdin