Develop Reference

Python make each lock in multithread

I'm considering how to make several locks for each thread.
I have 3 threads right now.
A : main thread(data sending)
B : data receiving thread
C : data sending every 2 sec thread
I don't want to stop B(receiving thread) except sending time.
How can I use Lock between A,B and between A,C easily!!...
class A:
def __init__():
self._A_B_lock = RLock()
self._A_C_lock = RLock()
self._B = threading.Thread(target=B_receiving_thread, args=(self._A_B_lock,) ... ).start()
self._C = threading.Thread(target=C_sending_2sec_thread, args=(self._A_C_lock,) ... ).start()
def sending():
with A_B_lock:
sending_data()
def B_receiving_thread(self,A_B_lock):
while(1):
with A_B_lock:
receiving_data()
#do something
def C_sending_2sec_thread(self,A_C_lock):
while(1):
with A_C_lock:
self.sending()
# actually I want to make decorator with A_C_lock, I have so many functions.
def so_many_functions():
with self.A_C_lock:
#do important thing
This code doesn’t work..

A decorator is a good idea. You could use this
def decorator(*locks):
def _decorator(func):
def inner_function(*args, **kwargs):
for lock in locks:
lock.acquire()
value = func(*args, **kwargs)
for lock in locks:
lock.release()
return value
return inner_function
return _decorator
And then you decorate each function, and pass as parameter all the locks that that function will need to make his job without interfeering others. Like this,
lock1 = threading.Lock()
lock2 = threading.Lock()
#decorator(lock1, lock2)
def f1(word):
for char in word:
print(char)
'''DO STUFF'''
#decorator(lock1, lock2)
def f2(word):
for char in word:
print(char)
'''
DO STUFF
'''
t1 = threading.Thread(target=f1, args=('Hello ',))
t2 = threading.Thread(target=f2, args=('world',))
t1.start()
t2.start()
Thats just a dummy example, but you can easily apply it to your code. Good thing about it, is that you can easily choose wich locks you want to use for each different function.
Hope it helps

Is there any replacement for empty while loops?

I'm using empty while loops a lot, for example:
I have a thread running in the background that will change a value called "a" in 5 seconds. however, I'm using a different function at the same time, and I want to let the second function know that the value has changed, so what I always did was:
import threading, time
class example:
def __init__(self):
self.a = 0
def valchange(self):
time.sleep(5)
self.a += 1
time.sleep(1)
print("im changing the a value to " + str(self.a))
print("those print commands needs to run after notifier stopped his while and started printing")
def notifier(exam :example, num :int):
while(exam.a != num):
pass
print("it changed to " + str(num))
exa = example()
i = 1
while(i <= 16):
temp= threading.Thread(target=notifier, args=(exa, i, ))
temp.start()
i += 3
i = 1
while(i <= 16):
exa.valchange()
i += 1
It's important to mention, that example could not use wait and set to an event, because there is no indication to when you need to run set, and how much threads are running in the background, and even what numbers will have a thread waiting for them to change.
And also you can't use join because changing 'a' is not a sign to print, only the condition is the sign.
Async and select can't help me as well because of the last reason.
Is there any way to create something, that will stop the program fromrunning until the condition will become true? you can provide your solution with any programming language you want, but mainly I'm using python 3.
EDIT: please remember that I need it to work with every condition. And my code example- is only an example, so if something works there, it doesn't necessarily will work with a different condition.
Thank you very much in advance :)
Idea:
wait(a == 5) // will do nothing until a == 5

You need to use select or epoll system calls if you're waiting for some system operation to finish. In case you're waiting for a certain IO event, then you can use asyncio (provided your Python version > 3.3), otherwise you could consider twisted.
If you're doing some CPU bound operations you need to consider multiple processes or threads, only then you can do any such monitoring effectively. Having a while loop running infinitely without any interruption is a disaster waiting to happen.

If your thread only changes a's value once, at the end of its life, then you can use .join() to wait for the thread to terminate.
import threading
import time
class example:
def __init__(self):
self.a = 0
self.temp = threading.Thread(target=self.valchange)
self.temp.start()
self.notifier()
def valchange(self):
time.sleep(5)
self.a = 1
def notifier(self):
self.temp.join()
print("the value of a has changed")
example()
If the thread might change a's value at any point in its lifetime, then you can use one of the threading module's more generalized control flow objects to coordinate execution. For instance, the Event object.
import threading
import time
class example:
def __init__(self):
self.a = 0
self.event = threading.Event()
temp = threading.Thread(target=self.valchange)
temp.start()
self.notifier()
def valchange(self):
time.sleep(5)
self.a = 1
self.event.set()
def notifier(self):
self.event.wait()
print("the value of a has changed")
example()
One drawback to this Event approach is that the thread target has to explicitly call set() whenever it changes the value of a, which can be irritating if you change a several times in your code. You could automate this away using a property:
import threading
import time
class example(object):
def __init__(self):
self._a = 0
self._a_event = threading.Event()
temp = threading.Thread(target=self.valchange)
temp.start()
self.notifier()
#property
def a(self):
return self._a
#a.setter
def a(self, value):
self._a = value
self._a_event.set()
def valchange(self):
time.sleep(5)
self.a = 1
def notifier(self):
self._a_event.wait()
print("the value of a has changed")
example()
Now valchange doesn't have to do anything special after setting a's value.

What you are describing is a spin lock, and might be fine, depending on your use case.
The alternative approach is to have the code you are waiting on call you back when it reaches a certain condition. This would require an async framework such as https://docs.python.org/3/library/asyncio-task.html
There are some nice simple examples in those docs so I won't insult your intelligence by pasting them here.

Multithreading (?): Manual interference in a loop

I've been looking into a way to directly change variables in a running module.
What I want to achieve is that a load test is being run and that I can manually adjust the call pace or whatsoever.
Below some code that I just created (not-tested e.d.), just to give you an idea.
class A():
def __init__(self):
self.value = 1
def runForever(self):
while(1):
print self.value
def setValue(self, value):
self.value = value
if __name__ == '__main__':
#Some code to create the A object and directly apply the value from an human's input
a = A()
#Some parallelism or something has to be applied.
a.runForever()
a.setValue(raw_input("New value: "))
Edit #1: Yes, I know that now I will never hit the a.setValue() :-)

Here is a multi-threaded example. This code will work with the python interpreter but not with the Python Shell of IDLE, because the raw_input function is not handled the same way.
from threading import Thread
from time import sleep
class A(Thread):
def __init__(self):
Thread.__init__(self)
self.value = 1
self.stop_flag = False
def run(self):
while not self.stop_flag:
sleep(1)
print(self.value)
def set_value(self, value):
self.value = value
def stop(self):
self.stop_flag = True
if __name__ == '__main__':
a = A()
a.start()
try:
while 1:
r = raw_input()
a.set_value(int(r))
except:
a.stop()

The pseudo code you wrote is quite similar to the way Threading / Multiprocessing works in python. You will want to start a (for example) thread that "runs forever" and then instead of modifying the internal rate value directly, you will probably just send a message through a Queue that gives the new value.
Check out this question.
Here is a demonstration of doing what you asked about. I prefer to use Queues to directly making calls on threads / processes.
import Queue # !!warning. if you use multiprocessing, use multiprocessing.Queue
import threading
import time
def main():
q = Queue.Queue()
tester = Tester(q)
tester.start()
while True:
user_input = raw_input("New period in seconds or (q)uit: ")
if user_input.lower() == 'q':
break
try:
new_speed = float(user_input)
except ValueError:
new_speed = None # ignore junk
if new_speed is not None:
q.put(new_speed)
q.put(Tester.STOP_TOKEN)
class Tester(threading.Thread):
STOP_TOKEN = '<<stop>>'
def __init__(self, q):
threading.Thread.__init__(self)
self.q = q
self.speed = 1
def run(self):
while True:
# get from the queue
try:
item = self.q.get(block=False) # don't hang
except Queue.Empty:
item = None # do nothing
if item:
# stop when requested
if item == self.STOP_TOKEN:
break # stop this thread loop
# otherwise check for a new speed
try:
self.speed = float(item)
except ValueError:
pass # whatever you like with unknown input
# do your thing
self.main_code()
def main_code(self):
time.sleep(self.speed) # or whatever you want to do
if __name__ == '__main__':
main()

Python threading. How do I lock a thread?

I'm trying to understand the basics of threading and concurrency. I want a simple case where two threads repeatedly try to access one shared resource.
The code:
import threading
class Thread(threading.Thread):
def __init__(self, t, *args):
threading.Thread.__init__(self, target=t, args=args)
self.start()
count = 0
lock = threading.Lock()
def increment():
global count
lock.acquire()
try:
count += 1
finally:
lock.release()
def bye():
while True:
increment()
def hello_there():
while True:
increment()
def main():
hello = Thread(hello_there)
goodbye = Thread(bye)
while True:
print count
if __name__ == '__main__':
main()
So, I have two threads, both trying to increment the counter. I thought that if thread 'A' called increment(), the lock would be established, preventing 'B' from accessing until 'A' has released.
Running the makes it clear that this is not the case. You get all of the random data race-ish increments.
How exactly is the lock object used?
Additionally, I've tried putting the locks inside of the thread functions, but still no luck.

You can see that your locks are pretty much working as you are using them, if you slow down the process and make them block a bit more. You had the right idea, where you surround critical pieces of code with the lock. Here is a small adjustment to your example to show you how each waits on the other to release the lock.
import threading
import time
import inspect
class Thread(threading.Thread):
def __init__(self, t, *args):
threading.Thread.__init__(self, target=t, args=args)
self.start()
count = 0
lock = threading.Lock()
def incre():
global count
caller = inspect.getouterframes(inspect.currentframe())[1][3]
print "Inside %s()" % caller
print "Acquiring lock"
with lock:
print "Lock Acquired"
count += 1
time.sleep(2)
def bye():
while count < 5:
incre()
def hello_there():
while count < 5:
incre()
def main():
hello = Thread(hello_there)
goodbye = Thread(bye)
if __name__ == '__main__':
main()
Sample output:
...
Inside hello_there()
Acquiring lock
Lock Acquired
Inside bye()
Acquiring lock
Lock Acquired
...

import threading
# global variable x
x = 0
def increment():
"""
function to increment global variable x
"""
global x
x += 1
def thread_task():
"""
task for thread
calls increment function 100000 times.
"""
for _ in range(100000):
increment()
def main_task():
global x
# setting global variable x as 0
x = 0
# creating threads
t1 = threading.Thread(target=thread_task)
t2 = threading.Thread(target=thread_task)
# start threads
t1.start()
t2.start()
# wait until threads finish their job
t1.join()
t2.join()
if __name__ == "__main__":
for i in range(10):
main_task()
print("Iteration {0}: x = {1}".format(i,x))

Is there an easy way in Python to wait until certain condition is true?

I need to wait in a script until a certain number of conditions become true?
I know I can roll my own eventing using condition variables and friends, but I don't want to go through all the trouble of implementing it, since some object property changes come from external thread in a wrapped C++ library (Boost.Python), so I can't just hijack __setattr__ in a class and put a condition variable there, which leaves me with either trying to create and signal a Python condition variable from C++, or wrap a native one and wait on it in Python, both of which sound fiddly, needlessly complicated and boring.
Is there an easier way to do it, barring continuous polling of the condition?
Ideally it would be along the lines of
res = wait_until(lambda: some_predicate, timeout)
if (not res):
print 'timed out'

Unfortunately the only possibility to meet your constraints is to periodically poll, e.g....:
import time
def wait_until(somepredicate, timeout, period=0.25, *args, **kwargs):
mustend = time.time() + timeout
while time.time() < mustend:
if somepredicate(*args, **kwargs): return True
time.sleep(period)
return False
or the like. This can be optimized in several ways if somepredicate can be decomposed (e.g. if it's known to be an and of several clauses, especially if some of the clauses are in turn subject to optimization by being detectable via threading.Events or whatever, etc, etc), but in the general terms you ask for, this inefficient approach is the only way out.

Another nice package is waiting - https://pypi.org/project/waiting/
install:
pip install waiting
Usage:
You pass a function that will be called every time as a condition, a timeout, and (this is useful) you can pass a description for the waiting, which will be displayed if you get TimeoutError.
using function:
from waiting import wait
def is_something_ready(something):
if something.ready():
return True
return False
# wait for something to be ready
something = # whatever
wait(lambda: is_something_ready(something), timeout_seconds=120, waiting_for="something to be ready")
# this code will only execute after "something" is ready
print("Done")
Note: the function must return a boolean - True when the wait is over, False otherwise

Here is another solution. The goal was to make threads to wait on each other before doing some work in a very precise order. The work can take unknown amount of time. Constant polling is not good for two reasons: it eats CPU time and action does not start immediately after condition is met.
class Waiter():
def __init__(self, init_value):
self.var = init_value
self.var_mutex = threading.Lock()
self.var_event = threading.Event()
def WaitUntil(self, v):
while True:
self.var_mutex.acquire()
if self.var == v:
self.var_mutex.release()
return # Done waiting
self.var_mutex.release()
self.var_event.wait(1) # Wait 1 sec
def Set(self, v):
self.var_mutex.acquire()
self.var = v
self.var_mutex.release()
self.var_event.set() # In case someone is waiting
self.var_event.clear()
And the way to test it
class TestWaiter():
def __init__(self):
self.waiter = Waiter(0)
threading.Thread(name='Thread0', target=self.Thread0).start()
threading.Thread(name='Thread1', target=self.Thread1).start()
threading.Thread(name='Thread2', target=self.Thread2).start()
def Thread0(self):
while True:
self.waiter.WaitUntil(0)
# Do some work
time.sleep(np.random.rand()*2)
self.waiter.Set(1)
def Thread1(self):
while True:
self.waiter.WaitUntil(1)
# Do some work
time.sleep(np.random.rand())
self.waiter.Set(2)
def Thread2(self):
while True:
self.waiter.WaitUntil(2)
# Do some work
time.sleep(np.random.rand()/10)
self.waiter.Set(0)
Waiter for multiprocessing:
import multiprocessing as mp
import ctypes
class WaiterMP():
def __init__(self, init_value, stop_value=-1):
self.var = mp.Value(ctypes.c_int, init_value)
self.stop_value = stop_value
self.event = mp.Event()
def Terminate(self):
self.Set(self.stop_value)
def Restart(self):
self.var.value = self.init_value
def WaitUntil(self, v):
while True:
if self.var.value == v or self.var.value == self.stop_value:
return
# Wait 1 sec and check aiagn (in case event was missed)
self.event.wait(1)
def Set(self, v):
exit = self.var.value == self.stop_value
if not exit: # Do not set var if threads are exiting
self.var.value = v
self.event.set() # In case someone is waiting
self.event.clear()
Please comment if this is still not the best solution.

You've basically answered your own question: no.
Since you're dealing with external libraries in boost.python, which may change objects at their leisure, you need to either have those routines call an event handler refresh, or work with a condition.

Here is the threading extention to Alex's solution:
import time
import threading
# based on https://stackoverflow.com/a/2785908/1056345
def wait_until(somepredicate, timeout, period=0.25, *args, **kwargs):
must_end = time.time() + timeout
while time.time() < must_end:
if somepredicate(*args, **kwargs):
return True
time.sleep(period)
return False
def wait_until_par(*args, **kwargs):
t = threading.Thread(target=wait_until, args=args, kwargs=kwargs)
t.start()
print ('wait_until_par exits, thread runs in background')
def test():
print('test')
wait_until_par(test, 5)

From the computational perspective there must be a check for all conditions somewhere, sometime. If you have two parts of code, one that generates conditions changes and the other one that should be executed when some are true, you can do the following:
Have the code that changes conditions in, say, main thread, and the code that should be launched when some conditions are true, in a worker thread.
from threading import Thread,Event
locker = Event()
def WhenSomeTrue(locker):
locker.clear() # To prevent looping, see manual, link below
locker.wait(2.0) # Suspend the thread until woken up, or 2s timeout is reached
if not locker.is_set(): # when is_set() false, means timeout was reached
print('TIMEOUT')
else:
#
# Code when some conditions are true
#
worker_thread = Thread(target=WhenSomeTrue, args=(locker,))
worker_thread.start()
cond1 = False
cond2 = False
cond3 = False
def evaluate():
true_conditions = 0
for i in range(1,4):
if globals()["cond"+str(i)]: #access a global condition variable one by one
true_conditions += 1 #increment at each true value
if true_conditions > 1:
locker.set() # Resume the worker thread executing the else branch
#Or just if true_conditions > 1: locker.set();
#true_conditions would need be incremented when 'True' is written to any of those variables
#
# some condition change code
#
evaluate()
For more information concerning this method, visit: https://docs.python.org/3/library/threading.html#event-objects

Proposed solution:
def wait_until(delegate, timeout: int):
end = time.time() + timeout
while time.time() < end:
if delegate():
return True
else:
time.sleep(0.1)
return False
Usage:
wait_until(lambda: True, 2)

I once used this in my code:
while not condition:
pass
Hope this helps

In 2022 now you could use https://trio-util.readthedocs.io/en/latest/#trio_util.AsyncValue
I think this comes closest to what you want in its "smoothest" form

This worked for me
direction = ''
t = 0
while direction == '' and t <= 1:
sleep(0.1)
t += 0.1
This is for waiting for a signal while making sure time limit of 1 second

here's how:
import time
i = false
while i == false:
if (condition):
i = true
break

Here's my Code I used during one of my Projects :
import time
def no() :
if (Condition !!!) :
it got true
oh()
else:
time.sleep(1) /Don't remove or don't blame me if ur system gets ""DEAD""
no()
def oh() : /Ur main program
while True:
if(bla) :
.......
no()
else :
time.sleep(1)
oh()
oh()
Hope it Helps