I'm trying to utilize threading and queueing (based on a recommendation) to pause the main process.
My program basically iterates through images, opening and closing them utilizing a 3-second time-loop for each iteration.
I'm trying to use threading to interject a time.sleep(20) if a certain condition is met (x == True). The condition is being met (evident by the output of the print statement), but time.sleep(20) is not affecting the main process.
I plan to subsitute time.sleep(20) with a more complex process but for simpliclity I've used it here.
import time
import subprocess
import pickle
import keyboard
import threading
from threading import Thread
import multiprocessing
import queue
import time
with open('C:\\Users\Moondra\\Bioteck.pickle', 'rb') as file:
bio = pickle.load(file)
q = queue.LifoQueue(0)
def keyboard_press(): # This is just receiving boolean values based on key presses
while True:
q.put(keyboard.is_pressed('down'))
x = q.get()
print(x)
if x == True:
time.sleep(20)
t = Thread(target = keyboard_press, args= ())
t.start()
if __name__ == "__main__":
for i in bio[:5]:
p = subprocess.Popen(["C:\Program Files\IrfanView\i_view64.exe",'C:\\Users\Moondra\\Bioteck_charts\{}.png'.format(i)])
time.sleep(3)
p.kill()
So why isn't my thread affecting my main process?
Thank you.
Update:
So It seems I have to use flags and use flag as a global variable within my function. I would like to avoid using global but it's not working without globalizing flag within my function.
Second, I don't know how to restart the thread.
Once the thread returns the flag as false, the thread sort of just stalls.
I tried starting the thread again, with t.start, but I received the error:
RuntimeError: threads can only be started once
Here is updated code:
def keyboard_press():
while True:
global flag
q.put(keyboard.is_pressed('down'))
x = q.get()
print(x)
if x == True:
flag = False
#print('keyboard_flag is',flag)
return flag
if __name__ == "__main__":
flag = True
q = queue.LifoQueue(0)
t = Thread(target = keyboard_press, args= ())
t.start()
for i in bio[:5]:
p = subprocess.Popen(["C:\Program Files\IrfanView\i_view64.exe",'C:\\Users\Moondra\\Bioteck_charts\{}.png'.format(i)])
time.sleep(3)
print ('flag is',flag)
if flag == True:
p.kill()
else:
time.sleep(20)
p.kill()
flag = True
#t.start() #doesn't seem to work.
why isn't my thread affecting my main process?
Because you have not written any code to be executed by the keyboard_press() thread that would affect the main process.
It looks like you're trying to create a slide show that shows one image every three seconds, and you want it to pause for an extra twenty seconds when somebody presses a key. Is that right?
So, you've got one thread (the main thread) that runs the slide show, and you've got another that polls the keyboard, but your two threads don't communicate with one another.
You put a time.sleep(20) call in your keyboard thread. But that only pauses the keyboard thread. It doesn't do anything at all to the main thread.
What you need, is for the keyboard thread to set a variable that the main thread looks at after it wakes up from its three second sleep. The main thread can look at the variable, and see if a longer sleep has been requested, and if so, sleep for twenty more seconds.
Of course, after the longer sleep, you will want the main thread to re-set the variable so that it won't always sleep for twenty seconds after the first time the keyboard is touched.
P.S.: I am not a Python expert. I know that in other programming environments (e.g., Java), you also have to worry about "memory visibility." That is, when a variable is changed by one thread, there is no guarantee of when (if ever) some other thread will see the change...
...Unless, the threads use some kind of synchronization when they access the variable.
Based on what I have read (It's on the Internet! It must be true!), Python either does not have that problem now, or it did not have that problem in the recent past. I'm not sure which.
If memory consistency actually is an issue, then you will either have to use a mutex when you access the shared variable, or else you will have to make the threads communicate through some kind of a synchronized object such as a queue.
Related
I'm creating 5 threads for handling various tasks such as reading from sensors (Raspberry Pi), TCP connections and recently recording audio (pyAudio).
I am instantiating all threads in main() identically e.g.:
if __name__ == '__main__':
main()
def main():
global network_thread
network_thread = threading.Thread(name="NET-CONN", target=network_thread_run, args=())
network_thread.start()
I keep a global reference so I can kill the threads at shutdown with join().
Now, I have added thread #5:
global audio_thread
audio_thread = threading.Thread(name="AUDIO", target=audio_thread_run(), args=())
audio_thread.start()
...but my logging indicates it's running on the MainThread. I also double-checked inside the audio_thread_run() function and it is indeed running on MainThread:
if threading.current_thread() is threading.main_thread():
logger.warning("Audio thread is the same as MainThread!")
Why is this thread running on the MainThread? Have I hit a limit on the Pi?
Let's have a look at the two places where you create threads, modified slightly so they'll fit on one line, and with white-space inserted so they line up:
net_thread = threading.Thread(name="NET", target=net_run , args=())
aud_thread = threading.Thread(name="AUD", target=aud_run(), args=())
# Hmmm, what's this suspicious-looking thing here? ---->^^
Enough fun :-) The problem is that you are actually calling the audio_thread_run() function directly from your main thread and presumably, if it ever returned, you would then try to use the result as a callable to start a thread.
If you actually got rid of the thread start stuff altogether, it would boil down to the much simpler:
audio_thread_run()
which will very much run that function from the context of the main thread.
What you need to do is to remove the parentheses so it matches what you've down with the network threads:
audio_thread = threading.Thread(name="AUDIO", target=audio_thread_run, args=())
Not sure whether it's the right way to ask. I will just show you guys some code.
Say I write some a simple python script:
#!/usr/bin/python3
import threading
from time import sleep
def echo_num(num):
sleep(1)
print('the number is', num)
if __name__ == '__main__':
threads = []
for i in range(10):
t = threading.Thread(target=echo_num, args=(i,), daemon=None)
threads.append(t)
for t in threads:
t.start()
the script exits after all the threads end. After reading the threading-doc I found:
A thread can be flagged as a “daemon thread”. The significance of this flag is that the entire Python program exits when only daemon threads are left. The initial value is inherited from the creating thread.
If the script is daemon(therefore the called threads are also daemon), the called threads should not print anything.
So I assume the python script is by default not daemon?
From the documentation you linked, emphasis mine:
daemon
A boolean value indicating whether this thread is a daemon thread (True) or not (False). This must be set before start() is called, otherwise RuntimeError is raised. Its initial value is inherited from the creating thread; the main thread is not a daemon thread and therefore all threads created in the main thread default to daemon = False.
I have a simple watchdog in python 3 that reboots my server if something goes wrong:
import time, os
from multiprocessing import Pool
def watchdog(x):
time.sleep(x)
os.system('reboot')
return
def main():
while True:
p = Pool(processes=1)
p.apply_async(watchdog, (60, )) # start watchdog with 60s interval
# here some code thas has a little chance to block permanently...
# reboot is ok because of many other files running independently
# that will get problems too if this one blocks too long and
# this will reset all together and autostart everything back
# block is happening 1-2 time a month, mostly within a http-request
p.terminate()
p.join()
return
if __name__ == '__main__':
main()
p = Pool(processes=1) is declared every time the while loop starts.
Now here the question: Is there any smarter way?
If I p.terminate() to prevent the process from reboot, the Pool becomes closed for any other work. Or is there even nothing wrong with declaring a new Pool every time because of garbage collection.
Use a process. Processes support all of the features you are using, so you don't need to make a pool with size one. While processes do have a warning about using the terminate() method (since it can corrupt pipes, sockets, and locking primitives), you are not using any of those items and don't need to care. (In any event, Pool.terminate() probably has the same issues with pipes etc. even though it lacks a similar warning.)
I have a background thread that main calls, the background thread can open a number of different scripts but occasionally it will get an infinite print loop like this.
In thing.py
import foo
def main():
thr = Thread(target=background)
thr.start()
thread_list.append(thr)
def background():
getattr(foo, 'bar')()
return
And then in foo.py
def bar():
while True:
print("stuff")
This is what it's supposed to do but I want to be able to kill it when I need to. Is there a way for me to kill the background thread and all the functions it has called? I've tried putting flags in background to return when the flag goes high, but background is never able to check the flags since its waiting for bar to return.
EDIT: foo.py is not my code so I'm hesitant to edit it, ideally I could do this without modifying foo.py but if its impossible to avoid its okay
First of all it is very difficult (if possible) to control threads from other threads, no matter what language you are using. This is due to potential security issues. So what you do is you create a shared object which both threads can freely access. You can set a flag on it.
But luckily in Python each thread has its own Thread object which we can use:
import foo
def main():
thr = Thread(target=background)
thr.exit_requested = False
thr.start()
thread_list.append(thr)
def background():
getattr(foo, 'bar')()
return
And in foo:
import threading
def bar():
th = threading.current_thread()
# What happens when bar() is called from the main thread?
# The commented code is not thread safe.
# if not hasattr(th, 'exit_requested'):
# th.exit_requested = False
while not th.exit_requested:
print("stuff")
Although this will probably be hard to maintain/debug. Treat it more like a hack. Cleaner way would be to create a shared object and pass it around to all calls.
I have a code that calls to threads over a loop, something like this:
def SubmitData(data):
# creating the relevant command to execute
command = CreateCommand(data)
subprocess.call(command)
def Main():
while(True):
# generating some data
data = GetData()
MyThread = threading.Thread(target=SubmitData,args=(data,))
MyThread.start()
obviously, I don't use join() on the threads.
My question is how to join() those threads without making the main thread wait for them?
Do I even need to join() them? what will happend if I won't join() them?
some important points:
the while loop is suppose to for a very long time (couple of days)
the command itself is not very long (few seconds)
I'm using threading for Performance so if someone have a better idea instead, I would like to try it out.
Popen() doesn't block. Unless CreateCommand() blocks, you could call SubmitData() in the main thread:
from subprocess import Popen
processes = []
while True:
processes = [p for p in processes if p.poll() is None] # leave only running
processes.append(Popen(CreateCommand(GetData()))) # start a new one
Do I even need to join() them? what will happend if I won't join() them?
No. You don't need to join them. All non-daemonic threads are joined automatically when the main thread exits.