Develop Reference

How to use pyserial with a device connected with a USB port?

I am currently trying to use pyserial to read the values from my handheld tachometer, the specific model is the DT-2100.
I am using python 3 and my current code looks like this:
# Imports
import serial
import time
import io
# Coding section
# Setting Parameters
port = "COM3"
baud = 38400
data = []
info = 0
# Setting the port location, baudrate, and timeout value
ser = serial.Serial(port, baud, timeout=2)
# Ensuring that the port is open
if ser.isOpen():
print(ser.name + ' is open...')
# trying to read a single value from the display
#input("Ensure that the DT-2100 is turned on...")
info = ser.write(b'CSD')
ser.write(b'CSD')
info_real = ser.readlines()
print()
print("The current value on the screen is: ", info)
print()
print("The real value on the screen is: ", info_real)
This is what I get back after running the code:
COM3 is open...
The current value on the screen is: 3
The real value on the screen is: []
Process finished with exit code 0
The main issue is that I should be getting the value that is displayed by the tachometer, which for this test was 0, but between my two attempted methods I got 3 and nothing.
Any help is greatly appreciated.

The zip file you linked to contained an xls file which seemed to detail all the commands.
All the commands seem to be wrapped in: <STX> cmd <CR>, so you are missing those.
The CSD command would need to be like this: ser.write(b'\x02CSD\r')
Similarly the reply is also wrapped in the same way and you would need to remove those bytes and interpret the rest.

How to check output of a sub process but also hide it? [duplicate]

NB. I have seen Log output of multiprocessing.Process - unfortunately, it doesn't answer this question.
I am creating a child process (on windows) via multiprocessing. I want all of the child process's stdout and stderr output to be redirected to a log file, rather than appearing at the console. The only suggestion I have seen is for the child process to set sys.stdout to a file. However, this does not effectively redirect all stdout output, due to the behaviour of stdout redirection on Windows.
To illustrate the problem, build a Windows DLL with the following code
#include <iostream>
extern "C"
{
__declspec(dllexport) void writeToStdOut()
{
std::cout << "Writing to STDOUT from test DLL" << std::endl;
}
}
Then create and run a python script like the following, which imports this DLL and calls the function:
from ctypes import *
import sys
print
print "Writing to STDOUT from python, before redirect"
print
sys.stdout = open("stdout_redirect_log.txt", "w")
print "Writing to STDOUT from python, after redirect"
testdll = CDLL("Release/stdout_test.dll")
testdll.writeToStdOut()
In order to see the same behaviour as me, it is probably necessary for the DLL to be built against a different C runtime than than the one Python uses. In my case, python is built with Visual Studio 2010, but my DLL is built with VS 2005.
The behaviour I see is that the console shows:
> stdout_test.py
Writing to STDOUT from python, before redirect
Writing to STDOUT from test DLL
While the file stdout_redirect_log.txt ends up containing:
Writing to STDOUT from python, after redirect
In other words, setting sys.stdout failed to redirect the stdout output generated by the DLL. This is unsurprising given the nature of the underlying APIs for stdout redirection in Windows. I have encountered this problem at the native/C++ level before and never found a way to reliably redirect stdout from within a process. It has to be done externally.
This is actually the very reason I am launching a child process - it's so that I can connect externally to its pipes and thus guarantee that I am intercepting all of its output. I can definitely do this by launching the process manually with pywin32, but I would very much like to be able to use the facilities of multiprocessing, in particular the ability to communicate with the child process via a multiprocessing Pipe object, in order to get progress updates. The question is whether there is any way to both use multiprocessing for its IPC facilities and to reliably redirect all of the child's stdout and stderr output to a file.
UPDATE: Looking at the source code for multiprocessing.Processs, it has a static member, _Popen, which looks like it can be used to override the class used to create the process. If it's set to None (default), it uses a multiprocessing.forking._Popen, but it looks like by saying
multiprocessing.Process._Popen = MyPopenClass
I could override the process creation. However, although I could derive this from multiprocessing.forking._Popen, it looks like I would have to copy a bunch of internal stuff into my implementation, which sounds flaky and not very future-proof. If that's the only choice I think I'd probably plump for doing the whole thing manually with pywin32 instead.

The solution you suggest is a good one: create your processes manually such that you have explicit access to their stdout/stderr file handles. You can then create a socket to communicate with the sub-process and use multiprocessing.connection over that socket (multiprocessing.Pipe creates the same type of connection object, so this should give you all the same IPC functionality).
Here's a two-file example.
master.py:
import multiprocessing.connection
import subprocess
import socket
import sys, os
## Listen for connection from remote process (and find free port number)
port = 10000
while True:
try:
l = multiprocessing.connection.Listener(('localhost', int(port)), authkey="secret")
break
except socket.error as ex:
if ex.errno != 98:
raise
port += 1 ## if errno==98, then port is not available.
proc = subprocess.Popen((sys.executable, "subproc.py", str(port)), stdout=subprocess.PIPE, stderr=subprocess.PIPE)
## open connection for remote process
conn = l.accept()
conn.send([1, "asd", None])
print(proc.stdout.readline())
subproc.py:
import multiprocessing.connection
import subprocess
import sys, os, time
port = int(sys.argv[1])
conn = multiprocessing.connection.Client(('localhost', port), authkey="secret")
while True:
try:
obj = conn.recv()
print("received: %s\n" % str(obj))
sys.stdout.flush()
except EOFError: ## connection closed
break
You may also want to see the first answer to this question to get non-blocking reads from the subprocess.

I don't think you have a better option than redirecting a subprocess to a file as you mentioned in your comment.
The way consoles stdin/out/err work in windows is each process when it's born has its std handles defined. You can change them with SetStdHandle. When you modify python's sys.stdout you only modify where python prints out stuff, not where other DLL's are printing stuff. Part of the CRT in your DLL is using GetStdHandle to find out where to print out to. If you want, you can do whatever piping you want in windows API in your DLL or in your python script with pywin32. Though I do think it'll be simpler with subprocess.

Alternatively - and I know this might be slightly off-topic, but helped in my case for the same problem - , this can be resolved with screen on Linux:
screen -L -Logfile './logfile_%Y-%m-%d.log' python my_multiproc_script.py
this way no need to implement all the master-child communication

I assume I'm off base and missing something, but for what it's worth here is what came to mind when I read your question.
If you can intercept all of the stdout and stderr (I got that impression from your question), then why not add or wrap that capture functionality around each of your processes? Then send what is captured through a queue to a consumer that can do whatever you want with all of the outputs?

In my situation I changed sys.stdout.write to write to a PySide QTextEdit. I couldn't read from sys.stdout and I didn't know how to change sys.stdout to be readable. I created two Pipes. One for stdout and the other for stderr. In the separate process I redirect sys.stdout and sys.stderr to the child connection of the multiprocessing pipe. On the main process I created two threads to read the stdout and stderr parent pipe and redirect the pipe data to sys.stdout and sys.stderr.
import sys
import contextlib
import threading
import multiprocessing as mp
import multiprocessing.queues
from queue import Empty
import time
class PipeProcess(mp.Process):
"""Process to pipe the output of the sub process and redirect it to this sys.stdout and sys.stderr.
Note:
The use_queue = True argument will pass data between processes using Queues instead of Pipes. Queues will
give you the full output and read all of the data from the Queue. A pipe is more efficient, but may not
redirect all of the output back to the main process.
"""
def __init__(self, group=None, target=None, name=None, args=tuple(), kwargs={}, *_, daemon=None,
use_pipe=None, use_queue=None):
self.read_out_th = None
self.read_err_th = None
self.pipe_target = target
self.pipe_alive = mp.Event()
if use_pipe or (use_pipe is None and not use_queue): # Default
self.parent_stdout, self.child_stdout = mp.Pipe(False)
self.parent_stderr, self.child_stderr = mp.Pipe(False)
else:
self.parent_stdout = self.child_stdout = mp.Queue()
self.parent_stderr = self.child_stderr = mp.Queue()
args = (self.child_stdout, self.child_stderr, target) + tuple(args)
target = self.run_pipe_out_target
super(PipeProcess, self).__init__(group=group, target=target, name=name, args=args, kwargs=kwargs,
daemon=daemon)
def start(self):
"""Start the multiprocess and reading thread."""
self.pipe_alive.set()
super(PipeProcess, self).start()
self.read_out_th = threading.Thread(target=self.read_pipe_out,
args=(self.pipe_alive, self.parent_stdout, sys.stdout))
self.read_err_th = threading.Thread(target=self.read_pipe_out,
args=(self.pipe_alive, self.parent_stderr, sys.stderr))
self.read_out_th.daemon = True
self.read_err_th.daemon = True
self.read_out_th.start()
self.read_err_th.start()
#classmethod
def run_pipe_out_target(cls, pipe_stdout, pipe_stderr, pipe_target, *args, **kwargs):
"""The real multiprocessing target to redirect stdout and stderr to a pipe or queue."""
sys.stdout.write = cls.redirect_write(pipe_stdout) # , sys.__stdout__) # Is redirected in main process
sys.stderr.write = cls.redirect_write(pipe_stderr) # , sys.__stderr__) # Is redirected in main process
pipe_target(*args, **kwargs)
#staticmethod
def redirect_write(child, out=None):
"""Create a function to write out a pipe and write out an additional out."""
if isinstance(child, mp.queues.Queue):
send = child.put
else:
send = child.send_bytes # No need to pickle with child_conn.send(data)
def write(data, *args):
try:
if isinstance(data, str):
data = data.encode('utf-8')
send(data)
if out is not None:
out.write(data)
except:
pass
return write
#classmethod
def read_pipe_out(cls, pipe_alive, pipe_out, out):
if isinstance(pipe_out, mp.queues.Queue):
# Queue has better functionality to get all of the data
def recv():
return pipe_out.get(timeout=0.5)
def is_alive():
return pipe_alive.is_set() or pipe_out.qsize() > 0
else:
# Pipe is more efficient
recv = pipe_out.recv_bytes # No need to unpickle with data = pipe_out.recv()
is_alive = pipe_alive.is_set
# Loop through reading and redirecting data
while is_alive():
try:
data = recv()
if isinstance(data, bytes):
data = data.decode('utf-8')
out.write(data)
except EOFError:
break
except Empty:
pass
except:
pass
def join(self, *args):
# Wait for process to finish (unless a timeout was given)
super(PipeProcess, self).join(*args)
# Trigger to stop the threads
self.pipe_alive.clear()
# Pipe must close to prevent blocking and waiting on recv forever
if not isinstance(self.parent_stdout, mp.queues.Queue):
with contextlib.suppress():
self.parent_stdout.close()
with contextlib.suppress():
self.parent_stderr.close()
# Close the pipes and threads
with contextlib.suppress():
self.read_out_th.join()
with contextlib.suppress():
self.read_err_th.join()
def run_long_print():
for i in range(1000):
print(i)
print(i, file=sys.stderr)
print('finished')
if __name__ == '__main__':
# Example test write (My case was a QTextEdit)
out = open('stdout.log', 'w')
err = open('stderr.log', 'w')
# Overwrite the write function and not the actual stdout object to prove this works
sys.stdout.write = out.write
sys.stderr.write = err.write
# Create a process that uses pipes to read multiprocess output back into sys.stdout.write
proc = PipeProcess(target=run_long_print, use_queue=True) # If use_pipe=True Pipe may not write out all values
# proc.daemon = True # If daemon and use_queue Not all output may be redirected to stdout
proc.start()
# time.sleep(5) # Not needed unless use_pipe or daemon and all of stdout/stderr is desired
# Close the process
proc.join() # For some odd reason this blocks forever when use_queue=False
# Close the output files for this test
out.close()
err.close()

Here is the simple and straightforward way for capturing stdout for multiprocessing.Process:
import app
import io
import sys
from multiprocessing import Process
def run_app(some_param):
sys.stdout = io.TextIOWrapper(open(sys.stdout.fileno(), 'wb', 0), write_through=True)
app.run()
app_process = Process(target=run_app, args=('some_param',))
app_process.start()
# Use app_process.termninate() for python <= 3.7.
app_process.kill()

Nonblocking fifo

How can I make a fifo between two python processes, that allow dropping of lines if the reader is not able to handle the input?
If the reader tries to read or readline faster then the writer writes, it should block.
If the reader cannot work as fast as the writer writes, the writer should not block. Lines should not be buffered (except one line at a time) and only the last line written should be received by the reader on its next readline attempt.
Is this possible with a named fifo, or is there any other simple way for achiving this?

The following code uses a named FIFO to allow communication between two scripts.
If the reader tries to read faster than the writer, it blocks.
If the reader cannot keep up with the writer, the writer does not block.
Operations are buffer oriented. Line oriented operations are not currently implemented.
This code should be considered a proof-of-concept. The delays and buffer sizes are arbitrary.
Code
import argparse
import errno
import os
from select import select
import time
class OneFifo(object):
def __init__(self, name):
self.name = name
def __enter__(self):
if os.path.exists(self.name):
os.unlink(self.name)
os.mkfifo(self.name)
return self
def __exit__(self, exc_type, exc_value, exc_traceback):
if os.path.exists(self.name):
os.unlink(self.name)
def write(self, data):
print "Waiting for client to open FIFO..."
try:
server_file = os.open(self.name, os.O_WRONLY | os.O_NONBLOCK)
except OSError as exc:
if exc.errno == errno.ENXIO:
server_file = None
else:
raise
if server_file is not None:
print "Writing line to FIFO..."
try:
os.write(server_file, data)
print "Done."
except OSError as exc:
if exc.errno == errno.EPIPE:
pass
else:
raise
os.close(server_file)
def read_nonblocking(self):
result = None
try:
client_file = os.open(self.name, os.O_RDONLY | os.O_NONBLOCK)
except OSError as exc:
if exc.errno == errno.ENOENT:
client_file = None
else:
raise
if client_file is not None:
try:
rlist = [client_file]
wlist = []
xlist = []
rlist, wlist, xlist = select(rlist, wlist, xlist, 0.01)
if client_file in rlist:
result = os.read(client_file, 1024)
except OSError as exc:
if exc.errno == errno.EAGAIN or exc.errno == errno.EWOULDBLOCK:
result = None
else:
raise
os.close(client_file)
return result
def read(self):
try:
with open(self.name, 'r') as client_file:
result = client_file.read()
except OSError as exc:
if exc.errno == errno.ENOENT:
result = None
else:
raise
if not len(result):
result = None
return result
def parse_argument():
parser = argparse.ArgumentParser()
parser.add_argument('-c', '--client', action='store_true',
help='Set this flag for the client')
parser.add_argument('-n', '--non-blocking', action='store_true',
help='Set this flag to read without blocking')
result = parser.parse_args()
return result
if __name__ == '__main__':
args = parse_argument()
if not args.client:
with OneFifo('known_name') as one_fifo:
while True:
one_fifo.write('one line')
time.sleep(0.1)
else:
one_fifo = OneFifo('known_name')
while True:
if args.non_blocking:
result = one_fifo.read_nonblocking()
else:
result = one_fifo.read()
if result is not None:
print result
The server checks if the client has opened the FIFO. If the client has opened the FIFO, the server writes a line. Otherwise, the server continues running. I have implemented a non-blocking read because the blocking read causes a problem: If the server restarts, most of the time the client stays blocked and never recovers. With a non-blocking client, a server restart is more easily tolerated.
Output
[user#machine:~] python onefifo.py
Waiting for client to open FIFO...
Waiting for client to open FIFO...
Writing line to FIFO...
Done.
Waiting for client to open FIFO...
Writing line to FIFO...
Done.
[user#machine:~] python onefifo.py -c
one line
one line
Notes
On startup, if the server detects that the FIFO already exists, it removes it. This is the easiest way to notify clients that the server has restarted. This notification is usually ignored by the blocking version of the client.

Well, that's not actually a FIFO (queue) as far as I am aware - it's a single variable. I suppose it might be implementable if you set up a queue or pipe with a maximum size of 1, but it seems that it would work better to use a Lock on a single object in one of the processes, which the other process references via a proxy object. The reader would set it to None whenever it reads, and the writer would overwrite the contents every time it writes.
You can get those to the other processes by passing the proxy of the object, and a proxy of the lock, as an argument to all relevant processes. To get it slightly more conveniently, you can use a Manager, which provides a single object with proxy that you can pass in, which contains and provides proxies for whatever other objects (including locks) you want to put in it. This answer provides a useful example of proper use of a Manager to pass objects into a new process.

A Python Wrapper or Handler for A Minecraft Server

I am using Windows and am looking for a handler or wrapper using Python for a Minecraft server so that I can automatically enter commands without user input. I have searched through many questions on the website and only found half answers (in my case at least). I believe I will need to use the subprocess module but cannot decide which to use at the moment I am experimenting with the Popen functions. I have found an answer which I modified for my case:
server = Popen("java -jar minecraft_server.jar nogui", stdin=PIPE, stdout=PIPE, stderr=STDOUT)
while True:
print(server.stdout.readline())
server.stdout.flush()
command = input("> ")
if command:
server.stdin.write(bytes(command + "\r\n", "ascii"))
server.stdin.flush()
This does work in some way but only prints a line for every time you enter a command, which cannot work and all my efforts to change this end up with the program unable to execute anything else and instead just read. This is not a duplicate question because none of the answers in similar questions could help me enough.

As you already know, your server.stdout.readline() and input("> ") are blocking your code execution.
You need to make your code non-blocking, by not waiting to actually return what you want, but by checking, if there is anything to read and ignore it, if there isn't and continue to do other things.
On Linux systems you might be able to use select module, but on Windows it only works on sockets.
I was able to make it work on Windows by using threads and queues. (note: it's Python 2 code)
import subprocess, sys
from Queue import Queue, Empty
from threading import Thread
def process_line(line):
if line == "stop\n": # lines have trailing new line characters
print "SERVER SHUTDOWN PREVENTED"
return None
elif line == "quit\n":
return "stop\n"
elif line == "l\n":
return "list\n"
return line
s = subprocess.Popen("java -jar minecraft_server.jar nogui", stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
def read_lines(stream, queue):
while True:
queue.put(stream.readline())
# terminal reading thread
q = Queue()
t = Thread(target=read_lines, args=(sys.stdin, q))
t.daemon = True
t.start()
# server reading thread
qs = Queue()
ts = Thread(target=read_lines, args=(s.stdout, qs))
ts.daemon = True
ts.start()
while s.poll() == None: # loop while the server process is running
# get a user entered line and send it to the server
try:
line = q.get_nowait()
except Empty:
pass
else:
line = process_line(line) # do something with the user entered line
if line != None:
s.stdin.write(line)
s.stdin.flush()
# just pass-through data from the server to the terminal output
try:
line = qs.get_nowait()
except Empty:
pass
else:
sys.stdout.write(line)
sys.stdout.flush()

python os.mkfifo() for Windows

Short version (if you can answer the short version it does the job for me, the rest is mainly for the benefit of other people with a similar task):
In python in Windows, I want to create 2 file objects, attached to the same file (it doesn't have to be an actual file on the hard-drive), one for reading and one for writing, such that if the reading end tries to read it will never get EOF (it will just block until something is written). I think in linux os.mkfifo() would do the job, but in Windows it doesn't exist. What can be done? (I must use file-objects).
Some extra details:
I have a python module (not written by me) that plays a certain game through stdin and stdout (using raw_input() and print). I also have a Windows executable playing the same game, through stdin and stdout as well. I want to make them play one against the other, and log all their communication.
Here's the code I can write (the get_fifo() function is not implemented, because that's what I don't know to do it Windows):
class Pusher(Thread):
def __init__(self, source, dest, p1, name):
Thread.__init__(self)
self.source = source
self.dest = dest
self.name = name
self.p1 = p1
def run(self):
while (self.p1.poll()==None) and\
(not self.source.closed) and (not self.source.closed):
line = self.source.readline()
logging.info('%s: %s' % (self.name, line[:-1]))
self.dest.write(line)
self.dest.flush()
exe_to_pythonmodule_reader, exe_to_pythonmodule_writer =\
get_fifo()
pythonmodule_to_exe_reader, pythonmodule_to_exe_writer =\
get_fifo()
p1 = subprocess.Popen(exe, shell=False, stdin=subprocess.PIPE, stdout=subprocess.PIPE)
old_stdin = sys.stdin
old_stdout = sys.stdout
sys.stdin = exe_to_pythonmodule_reader
sys.stdout = pythonmodule_to_exe_writer
push1 = Pusher(p1.stdout, exe_to_pythonmodule_writer, p1, '1')
push2 = Pusher(pythonmodule_to_exe_reader, p1.stdin, p1, '2')
push1.start()
push2.start()
ret = pythonmodule.play()
sys.stdin = old_stdin
sys.stdout = old_stdout

Following the two answers above, I accidentally bumped into the answer. os.pipe() does the job. Thank you for your answers.
I'm posting the complete code in case someone else is looking for this:
import subprocess
from threading import Thread
import time
import sys
import logging
import tempfile
import os
import game_playing_module
class Pusher(Thread):
def __init__(self, source, dest, proc, name):
Thread.__init__(self)
self.source = source
self.dest = dest
self.name = name
self.proc = proc
def run(self):
while (self.proc.poll()==None) and\
(not self.source.closed) and (not self.dest.closed):
line = self.source.readline()
logging.info('%s: %s' % (self.name, line[:-1]))
self.dest.write(line)
self.dest.flush()
def get_reader_writer():
fd_read, fd_write = os.pipe()
return os.fdopen(fd_read, 'r'), os.fdopen(fd_write, 'w')
def connect(exe):
logging.basicConfig(level=logging.DEBUG,\
format='%(message)s',\
filename=LOG_FILE_NAME,
filemode='w')
program_to_grader_reader, program_to_grader_writer =\
get_reader_writer()
grader_to_program_reader, grader_to_program_writer =\
get_reader_writer()
p1 = subprocess.Popen(exe, shell=False, stdin=subprocess.PIPE, stdout=subprocess.PIPE)
old_stdin = sys.stdin
old_stdout = sys.stdout
sys.stdin = program_to_grader_reader
sys.stdout = grader_to_program_writer
push1 = Pusher(p1.stdout, program_to_grader_writer, p1, '1')
push2 = Pusher(grader_to_program_reader, p1.stdin, p1, '2')
push1.start()
push2.start()
game_playing_module.play()
sys.stdin = old_stdin
sys.stdout = old_stdout
fil = file(LOG_FILE, 'r')
data = fil.read()
fil.close()
return data
if __name__=='__main__':
if len(sys.argv) != 2:
print 'Usage: connect.py exe'
print sys.argv
exit()
print sys.argv
print connect(sys.argv[1])

On Windows, you are looking at (Named or Anonymous) Pipes.
A pipe is a section of shared memory that processes use for communication. The process that creates a pipe is the pipe server. A process that connects to a pipe is a pipe client. One process writes information to the pipe, then the other process reads the information from the pipe.
To work with Windows Pipes, you can use Python for Windows extensions (pywin32), or the Ctypes module. A special utility module, win32pipe, provides an interface to the win32 pipe API's. It includes implementations of the popen[234]() convenience functions.
See how-to-use-win32-apis-with-python and similar SO questions (not specific to Pipes, but points to useful info).

For a cross-platform solution, I'd recommend building the file-like object on top of a socket on localhost (127.0.0.1) -- that's what IDLE does by default to solve a problem that's quite similar to yours.

os.pipe() returns an anonymous pipe, or a named pipe on Windows, which is very lightweight and efficient.
TCP sockets (as suggested by user1495323) are more heavyweight: you can see them with netstat for example, and each one requires a port number, and the number of available ports is limited to 64k per peer (e.g. 64k from localhost to localhost).
On the other hand, named pipes (on Windows) are limited because:
You can't use select() for nonblocking I/O on Windows, because they're not sockets.
There's no apparent way to read() with a timeout, and
Even making them non-blocking is difficult.
And sockets can be wrapped in Python-compatible filehandles using makefile(), which allows them to be used to redirect stdout or stderr. This makes this an attractive option for some use cases, such as sending stdout from one thread to another.
A socket can be constructed with an automatically-assigned port number like this (based on the excellent Python socket HOWTO):
with closing(socket.socket(socket.AF_INET, socket.SOCK_STREAM)) as input_socket:
# Avoid socket exhaustion by setting SO_REUSEADDR <https://stackoverflow.com/a/12362623/648162>:
input_socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
# localhost doesn't work if the definition is missing from the hosts file,
# and 127.0.0.1 only works with IPv4 loopback, but socket.gethostname()
# should always work:
input_socket.bind((socket.gethostname(), 0))
random_port_number = input_socket.getsockname()[1]
input_socket.listen(1)
# Do something with input_socket, for example pass it to another thread.
output_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# close() should not strictly be necessary here, but since connect() could fail, it avoids leaking fds
# in that case. "If a file descriptor is given, it is closed when the returned I/O object is closed".
with output_socket:
output_socket.connect((socket.gethostname(), random_port_number))
The user of input_socket (e.g. another thread) can then do:
with input_socket:
while True:
readables, _, _ = select.select([input_socket], [], [input_socket], 1.0)
if len(readables) > 0:
input_conn, addr = self.input_socket.accept()
break
with input_conn:
while True:
readables, _, errored = select.select([input_conn], [], [input_conn], 1.0)
if len(errored) > 0:
print("connection errored, stopping")
break
if len(readables) > 0:
read_data = input_conn.recv(1024)
if len(read_data) == 0:
print("connection closed, stopping")
break
else:
print(f"read data: {read_data!r}")