I have a GUI where I want to do multiprocessing. My problem comes when i want to share a variable from a class that I had created.
I am trying to share it into the two modules that are running simultaneosly by using class multiprocessing.Queue([maxsize]).
But it dosen't work...
sch=sched()
q=Queue()
q.put([sch])
def foo1():
sch=q.get([sch])
event=Event(8) #another class created
sch.add_list(event)
q.put([sch])
time.sleep(12)
def foo2():
time.sleep(4)
sch=q.get([sch])
q.put([sch])
print(sch.event_list)
if __name__ == '__main__':
p1 = Process(target=foo1)
p2 = Process(target=foo2)
p1.start()
p2.start()
You have to pass the Queue instance as argument to the processes.
from multiprocessing import Process, Queue
def foo1(q):
sch = q.get([sch])
event = Event(8)
sch.add_list(event)
q.put([sch])
time.sleep(12)
def foo2(q):
time.sleep(4)
sch = q.get([sch])
q.put([sch])
print(sch.event_list)
if __name__ == "__main__":
q = Queue()
sch = sched()
q.put([sch])
p1 = Process(target=foo1, args=(q,))
p2 = Process(target=foo2, args=(q,))
p1.start()
p2.start()
When Python forks a new process it gets its own namespace, so it is not possible to access variables of your main program from within a child process.
If you want to share one object without passing it around through Pipes or Queues, you can also use Python Manager classes (see https://docs.python.org/3/library/multiprocessing.html?#managers). A manager creates a shared object which can be accessed from child processes. I suppose, sch is a list, so I used manager.list() in my example. For other data types just check the Python docs.
from multiprocessing import Process, Manager
def foo():
sch = sch_shared # Read from shared list
if __name__ == "__main__":
sch = sched()
mgr = Manager()
sch_shared = mgr.list(sch) # Create a shared list and return proxy
p = Process(target=foo)
p.start()
Related
Lets assume that i am starting a process in python with the following code:
from multiprocessing import Process
import time
def f(name):
print ('hello ', name)
if __name__ == '__main__':
p = Process(target=f,name = "Process-1", args=('bob',))
p.start()
Now,i want to terminate the process.I can simply do:
p.terminate()
However, i would like to terminate the process by its name.Is that possible?
To do that, you need to store a map between your process objects and their names. Using an helper function it makes your code even easier to read (IMO):
def terminate(procname):
return pmap[procname].terminate()
if __name__ == '__main__':
pmap = {}
pname = "process-1"
p = Process(target=f,name = pname, args=('bob',))
pmap[pname] = p
p.start()
Then to terminate:
terminate(pname)
The end goal is to execute a method in background, but not in parallel : when multiple objects are calling this method, each should wait for their turn to proceed. To achieve running in background, I have to run the method in a subprocess (not a thread), and I need to start it using spawn (not fork). To prevent parallel executions, the obvious solution is to have a global lock shared between processes.
When processes are forked, which is the default on Unix, it is easy to achieve, as highlighted in both of the following codes.
We can share it as a class variable :
import multiprocessing as mp
from time import sleep
class OneAtATime:
l = mp.Lock()
def f(self):
with self.l:
sleep(1)
print("Hello")
if __name__ == "__main__":
a = OneAtATime()
b = OneAtATime()
p1 = mp.Process(target = a.f)
p2 = mp.Process(target = b.f)
p1.start()
p2.start()
Or we can pass it to the method :
import multiprocessing as mp
from time import sleep
class OneAtATime:
def f(self, l):
with l:
sleep(1)
print("Hello")
if __name__ == "__main__":
a = OneAtATime()
b = OneAtATime()
m = mp.Manager()
l = mp.Lock()
p1 = mp.Process(target = a.f, args = (l,))
p2 = mp.Process(target = b.f, args = (l,))
p1.start()
p2.start()
Both of these codes have the appropriate behaviour of printing "hello" at one second of interval.
However, when changing the start method to 'spawn', they become broken.
The first one (1) prints both "hello"s at the same time. This is because the internal state of a class is not pickled, so they do not have the same lock.
The second one (2) fails with FileNotFoundError at runtime. I think it has to do with the fact that locks cannot be pickled : see Python sharing a lock between processes.
In this answer, two fixes are suggested (side note : I cannot use a pool because I want to randomly create an arbitrary number of processes).
I haven't found a way to adapt the second fix, but I tried to implement the first one :
import multiprocessing as mp
from time import sleep
if __name__ == "__main__":
mp.set_start_method('spawn')
class OneAtATime:
def f(self, l):
with l:
sleep(1)
print("Hello")
if __name__ == "__main__":
a = OneAtATime()
b = OneAtATime()
m = mp.Manager()
l = m.Lock()
p1 = mp.Process(target = a.f, args = (l,))
p2 = mp.Process(target = b.f, args = (l,))
p1.start()
p2.start()
This fails with AttributeError and FileNotFoundError (3). In fact it also fails (BrokenPipe) when the fork method is used (4).
What is the proper way of sharing a lock between spawned processes ?
A quick explanation of the four fails I numbered would be nice, too.
I'm running Python 3.6 under Archlinux.
Congratulations, you got yourself 90% of the way there. The last step is actually not very hard to do.
Yes, your final code block fails with an AttributeError, but what specifically is the error? "Can't get attribute 'OneAtATime' on ". This is very similar to a problem you've already encountered - it's not pickling the class OneAtATime.
I made the following change and it worked as you'd like:
file ooat.py:
from time import sleep
class OneAtATime:
def f(self, l):
with l:
sleep(1)
print("Hello")
interactive shell:
import multiprocessing as mp
from oaat import OneAtATime
if __name__ == "__main__":
mp.set_start_method('spawn')
a = OneAtATime()
b = OneAtATime()
m = mp.Manager()
l = m.Lock()
p1 = mp.Process(target = a.f, args = (l,))
p2 = mp.Process(target = b.f, args = (l,))
p1.start()
p2.start()
You may notice, I didn't really do anything - just split your code into two separate files. Try it out, you'll see it works fine. (At least, it did for me, using python 3.5 on ubuntu.)
The last code snippet works, provided the script does not exit prematurely. Joining processes is enough :
import multiprocessing as mp
from time import sleep
class OneAtATime:
def f(self, l):
with l:
sleep(1)
print("Hello")
if __name__ == "__main__":
mp.set_start_method('spawn')
a = OneAtATime()
b = OneAtATime()
m = mp.Manager()
l = m.Lock()
p1 = mp.Process(target = a.f, args = (l,))
p2 = mp.Process(target = b.f, args = (l,))
p1.start()
p2.start()
p1.join()
p2.join()
More info on the error it was causing here https://stackoverflow.com/a/25456494/8194503.
I would like to have main python process to create a child process that continuosly updates an object (Node). An object needs to be accessible from both main process and child process. Once I add instance of my Node object to instance of manager.dict() when trying to retrieve Node object from it, main process is blocked.
Below is a simplified code
test.py
from multiprocessing import Process, Manager
import time
class Node(object):
def __init__(self, host):
self.host = host
self.refreshed = 0
def refresh(self):
self.refreshed = int(time.time())
def __repr__(self):
return 'Node host:%s' % (self.host,)
man = Manager()
d = man.dict()
def worker(d):
while True:
node = d['n1']
node.refresh()
d['n1'] = node
time.sleep(3)
proc = Process(target=worker, args=(d,))
run.py
import test
test.d['n1'] = test.Node('localhost')
test.proc.start()
If I drop to interpreter here and do test.d.items() it will block.
Update
If I alter the code and instead of Node instance just use primitive value, e.g. increment an int, it works fine.
Update
If I move code from run.py to the bottom of test.py so everything is in the same scope, then it works fine.
try to put your code behind
if __name__ == "main":
for example:
if __name__ == "__main__":
man = Manager()
d = man.dict()
proc = Process(target=worker, args=(d,))
I have a 3 processes running in one script. Process 1 passes data to Process 2, and then Process 2 passes data to Process 3. When I put data to queue2, error occurs that "Global name "queue2" is not defined", I am stuck on this error now...
if __name__ == '__main__':
queue1 = mp.Queue()
queue2 = mp.Queue()
p1 = mp.Process(target=f2, args=(queue1,))
p1.start()
p2 = mp.Process(target=f3, args=(queue2,))
p2.start()
f1()
def f1():
# do something to a get x
queue1.put(x)
def f2(q):
a = q.get()
# do something to a, to produce b
queue2.put(b) # error happens here: Global name "queue2" is not defined
def f3(q):
c = q.get()
# keeping processing c...
Just as you passed queue1 to f2, you also need to pass queue2.
You can declare the queues as global:
def f2(q):
global queue2
a = q.get()
queue2.put(b)
This works :
import multiprocessing as mp
queue1 = mp.Queue()
queue2 = mp.Queue()
def f1(q):
x = 5
# do something to a get x
q.put(x)
def f2(in_queue, out_queue):
a = in_queue.get()
b = a + 2
# do something to a, to produce b
out_queue.put(b)
def f3(q):
c = q.get()
print c
f1(queue1)
p1 = mp.Process(target=f2, args=(queue1, queue2))
p1.start()
p2 = mp.Process(target=f3, args=(queue2,))
p2.start()
Your code doesn't return the error you seem to have, it returns "f2 not defined" since you when you spawn the process p1, f2 is not a defined variable yet. The rule when you fork is that at creation time your processes must see the variables they use, i.e. they must be in the current scope.
To put it clearly, at spawning process time you inherit the current namespace from the parent process.
I am very new to Python, thus am possibly asking a simple question.
I am wrting a multiprocess code with Python:
from multiprocessing import Process
from multiprocessing import Queue
class myClass(object):
def __init__(self):
self.__i = 0
self.__name = 'rob'
return
def target_func(self, name, q):
self.__name = name
print 'Hello', self.__name
self.__i += 1
print self.__i
q.put([self.__i, self.__name])
return
def name(self):
return self.__name
def i(self):
return self.__i
if __name__ == '__main__':
mc = myClass()
q = Queue()
p = Process(target = mc.target_func, args = ('bob', q,))
p.start()
ret = q.get()
p.join()
p2 = Process(target = mc.target_func, args = ('tom', q,))
p2.start()
ret = q.get()
p2.join()
I expect the print out should be
Hello bob
1
Hello tom
2
But actually, the print out is
Hello bob
1
Hello tom
1 <------------------ Why it's not 2?
May I know what am I wrong?
Many thanks.
target_func is called in separated process. mc is copied to each subprocess; not shared between processes.
Using Thread, you will get expected(?) result. For safety you should use lock; I omitted it in following code.
from threading import Thread
from Queue import Queue
....
if __name__ == '__main__':
mc = myClass()
q = Queue()
p = Thread(target = mc.target_func, args = ('bob', q,))
p.start()
ret = q.get()
p.join()
p2 = Thread(target = mc.target_func, args = ('tom', q,))
p2.start()
ret = q.get()
p2.join()
Processes don't share memory, unlike threads. The name __i in the second process refers to a different variable, whose initial value was copied from the original process when you launched the subprocess.
You can use the Value or Array data types to transfer information from one process to another, or you can use the Queue to push data from the subprocess back the the original. All of these classes are included in the multiprocessing module
http://docs.python.org/2/library/multiprocessing.html#multiprocessing.Queue
http://docs.python.org/2/library/multiprocessing.html#multiprocessing.Value
http://docs.python.org/2/library/multiprocessing.html#multiprocessing.Array
The value of the variable is still the same since each process you create gets a full copy of the memory space of the parent process, including a copy of the mc class instance that you created earlier. Hence, when you modify the instance variable of mc from within each process, it does not affect the variable in your main process. Here's a more concise example of this behavior:
from multiprocessing import Process
class A(object):
def __init__(self):
self.var = 1
print "Initialized class: ",self
def test(self):
print self
print "Variable value:",self.var
self.var += 1
if __name__ == '__main__':
a = A()
p1 = Process(target = a.test)
#Creates a copy of the curent memory space and will print "Variable value: 1"
p1.start()
p2 = Process(target = a.test)
#Will still print "Variable value: 1"
p2.start()