I am running a subprocess using 'Popen'. I need to block till this subprocess finishes and then read its output.
p = Popen(command, stdin=PIPE, stdout=PIPE, stderr=PIPE, encoding="utf-8") p.communicate(): output = p.stdout.readline() print(output) I get an error that
ValueError: I/O operation on closed file. How can I read the output after the subprocess finishes, I do not want to use poll() though as the subprocess takes time and I would need to wait for its completion anyway.
This should work:
p = Popen(command, stdin=PIPE, stdout=PIPE, stderr=PIPE, encoding="utf-8") output, error = p.communicate() print(output) if error: print('error:', error, file=sys.stderr) However, subprocess.run() is preferred these days:
p = subprocess.run(command, stdout=subprocess.PIPE, stderr=subprocess.PIPE) print("output:", p.stdout) if proc.stderr: print("error:", p.stderr, file=sys.stderr) Use subprocess.check_output. It returns the output of the command.
actually when I wanted to run python3 -c "import time;time.sleep(3);print(1)" with subprocess.run or subprocess.Popen I faced that problem of not collecting outputs and returning before child process ended. The only feasible solution was/is for me asyncio code (for 3.6) below but for newer version will be pretty similar:
class ShellCommand(Command): OK = 0 ENCODING = 'utf-8' def run(self, args: List[str]) -> str: cmd = ' '.join(args) logging.info('executing %s', cmd) async def _run(cmd: str, future: asyncio.Future): proc = await asyncio.create_subprocess_shell( cmd, stdout=asyncio.subprocess.PIPE, stderr=asyncio.subprocess.STDOUT) stdout, stderr = await proc.communicate() if proc.returncode != type(self).OK: raise ValueError(f'Return code: {proc.returncode}', f'Message: {stderr.decode(type(self).ENCODING)}') future.set_result(stdout.decode(type(self).ENCODING)) asyncio.set_event_loop(asyncio.new_event_loop()) loop = asyncio.get_event_loop() future = asyncio.Future() asyncio.ensure_future(_run(cmd, future), loop=loop) loop.run_until_complete(future) loop.close() return future.result() 
p.communicate()returns the output.subprocess.run()or the legacysubprocess.check_output()? You should avoidPopenif you can precisely because it's tricky to get right.p.communicate()returnsbound method Popen.communicate of <subprocess.Popen object at 0x105e84e10, not the output of the subprocess codeoutput, error = p.communicate()should work, that output looks like you are just printingp.communicate(without the parentheses).