I am trying out the examples listed in the python docs http://docs.python.org/library/multiprocessing.html
particularly these two on Windows:
1)
from multiprocessing import Process
def f(name):
print 'hello', name
if __name__ == '__main__':
p = Process(target=f, args=('bob',))
p.start()
p.join()
2)
from multiprocessing import Process
import os
def info(title):
print title
print 'module name:', __name__
print 'parent process:', os.getppid()
print 'process id:', os.getpid()
def f(name):
info('function f')
print 'hello', name
if __name__ == '__main__':
info('main line')
p = Process(target=f, args=('bob',))
p.start()
p.join()
Here is the problem: I don't get any output from the child process. It works on Linux though. What is going on?
example 1 works well.( I hope you saved the program in a file and then executed it else it will not recognise the function f at all).
example 2 won't work if u want the parent process's id. There is no getppid in windows.
Just take the print os.getppid and execute, its brilliant as ever !
Please refer this for more by Doug. (UPDATE: The original link isn't working, here is something similar.)
Related
Is it possible to set the name of the Processes spawned by multiprocessing.Process or billiard.Process. SOmething like:
import billiard
for d in list:
processes.append(billiard.Process(target=evaluate))
for p in processes:
p.name = 'someID'
p.start()
I want to find those specific processes afterwards while they are running with:
import psutil
for proc in psutil.process_iter():
if proc.name() == 'someID':
print(proc)
If I may suggest, use process id instead of process name - thats way better, and would save you a whole lot of trouble.. since you just need a reference to the process later, use their ids instead of name.
Python 3.6 Official Docs have illustrated a very good way of playing around with process ids. Here is a snippet from the documentation
from multiprocessing import Process
import os
def info(title):
print(title)
print('module name:', __name__)
print('parent process:', os.getppid())
print('process id:', os.getpid())
def f(name):
info('function f')
print('hello', name)
if __name__ == '__main__':
info('main line')
p = Process(target=f, args=('bob',))
p.start()
p.join()
Also I see, Celery in your tags, if you're going to play with concurrent processes, I would recommend Supervisord instead. Gives you a very good control over the processes, if you're having a small scale project.
I have the codes like this:
It is clear the 'finished' has been printed out. but join still blocks.
Why should this happend?
from multiprocessing import Process
class MyProcess(Process):
def run(self):
## do someting
print 'finished'
processes = []
for i in range(3):
p = MyProcess()
p.start()
processes.append(p)
for p in processes:
p.join()
you should add this line if __name__ == '__main__': for things to work properly
Explanation:
your main script will be imported by process.py module, then it will execute your script lines 2 times, one during importing and one from your script execution,
here is the runtime error if we didn't include if __name__ == '__main__':
RuntimeError:
An attempt has been made to start a new process before the
current process has finished its bootstrapping phase.
This probably means that you are not using fork to start your
child processes and you have forgotten to use the proper idiom
in the main module:
if __name__ == '__main__':
freeze_support()
...
The "freeze_support()" line can be omitted if the program
is not going to be frozen to produce an executable.
your working code in python 3.6 is:
from multiprocessing import Process
class MyProcess(Process):
def run(self):
## do someting
print ('finished')
processes = []
if __name__ == '__main__':
for i in range(3):
p = MyProcess()
p.start()
processes.append(p)
for p in processes:
p.join()
print('we are done here .......')
output:
finished
finished
finished
we are done here .......
join would not block if the task is finished, also your program is invalid.
for i in 3: # X integer is not iterable,
for i in range(3): # should be like this.
I'm confused why the following block of code works the way it does. When a process is a daemon and doesn't call join() vs when it does call join(). When it doesn't call join(), it appears that the main process terminates and the daemon process both terminate after the main process terminates:
from multiprocessing import Process
import os
def info(title):
print(title)
print('module name:', __name__)
if hasattr(os, 'getppid'): # only available on Unix
print('parent process:', os.getppid())
print('process id:', os.getpid())
def f(name):
info('function f')
print('hello', name)
if __name__ == '__main__':
info('main line')
p = Process(target=f, args=('bob',))
p.daemon = True
p.start()
#p.join()
output:
main line
module name: __main__
parent process: 290
process id: 4793
join() is called:
from multiprocessing import Process
import os
def info(title):
print(title)
print('module name:', __name__)
if hasattr(os, 'getppid'): # only available on Unix
print('parent process:', os.getppid())
print('process id:', os.getpid())
def f(name):
info('function f')
print('hello', name)
if __name__ == '__main__':
info('main line')
p = Process(target=f, args=('bob',))
p.daemon = True
p.start()
p.join()
output:
main line
module name: __main__
parent process: 290
process id: 4807
function f
module name: __main__
parent process: 4807
process id: 4808
hello bob
Yes, you are right. When the main process terminates, the deamon process will also terminate.
This page will give you more details: Why is a Python multiprocessing daemon process not printing to standard output?
I am trying to learn how to use multiprocessingbut I can't get it to work. Here is the code right out of the documentation
from multiprocessing import Process
def f(name):
print 'hello', name
if __name__ == '__main__':
p = Process(target=f, args=('bob',))
p.start()
p.join()
it should output
>>> 'hello bob'
but instead i get
>>>
no errors or other messages, it just sits there, It is running in IDLE from a saved .py file on a Windows 7 machine with the 32-bit version of Python 2.7
My guess is that you are using IDLE to try to run this script. Unfortunately, this example will not run correctly in IDLE. Note the comment at the beginning of the docs:
Note Functionality within this package requires that the main
module be importable by the children. This is covered in Programming
guidelines however it is worth pointing out here. This means that some
examples, such as the multiprocessing.Pool examples will not work in
the interactive interpreter.
The __main__ module is not importable by children in IDLE, even if you run the script as a file with IDLE (which is commonly done with F5).
The problem is not IDLE. The problem is trying to print to sys.stdout in a process that has no sys.stdout. That is why Spyder has the same problem. Any GUI program on Windows is likely to have the same problem.
On Windows, at least, GUI programs are usually run in a process without stdin, stdout, or stderr streams. Windows expects GUI programs to interact with users through widgets that paint pixels on the screen (the G in Graphical) and receive key and mouse events from Windows event system. That is what the IDLE GUI does, using the tkinter wrapper of the tcl tk GUI framework.
When IDLE runs user code in a subprocess, idlelib.run runs first, and it replaces None for the standard streams with objects that interact with IDLE itself through a socket. Then it exec()s user code. When the user code runs multiprocessing, multiprocessing starts further processes that have no std streams, but never get them.
The solution is to start IDLE in a console: python -m idlelib.idle (the .idle is not needed on 3.x). Processes started in a console get std streams connect to the console. So do further subprocesses. The real stdout (as opposed to the sys.stdout) of all the processes is the console. If one runs the third example in the doc,
from multiprocessing import Process
import os
def info(title):
print(title)
print('module name:', __name__)
print('parent process:', os.getppid())
print('process id:', os.getpid())
def f(name):
info('function f')
print('hello', name)
if __name__ == '__main__':
info('main line')
p = Process(target=f, args=('bob',))
p.start()
p.join()
then the 'main line' block goes to the IDLE shell and the 'function f' block goes to the console.
This result shows that Justin Barber's claim that the user file run by IDLE cannot be imported into processes started by multiprocessing is not correct.
EDIT: Python saves the original stdout of a process in sys.__stdout__. Here is the result in IDLE's shell when IDLE is started normally on Windows, as a pure GUI process.
>>> sys.__stdout__
>>>
Here is the result when IDLE is started from CommandPrompt.
>>> import sys
>>> sys.__stdout__
<_io.TextIOWrapper name='<stdout>' mode='w' encoding='utf-8'>
>>> sys.__stdout__.fileno()
1
The standard file numbers for stdin, stdout, and stderr are 0, 1, 2. Run a file with
from multiprocessing import Process
import sys
def f(name):
print('hello', name)
print(sys.__stdout__)
print(sys.__stdout__.fileno())
if __name__ == '__main__':
p = Process(target=f, args=('bob',))
p.start()
p.join()
in IDLE started in the console and the output is the same.
It works.
I've marked the changes needed to make your sample run using comments:
from multiprocessing import Process
def f(name):
print 'hello', name #indent
if __name__ == '__main__':
p = Process(target=f, args=('bob',))
p.start()
p.join()` # remove ` (grave accent)
result:
from multiprocessing import Process
def f(name):
print 'hello', name
if __name__ == '__main__':
p = Process(target=f, args=('bob',))
p.start()
p.join()
Output from my laptop after saving it as ex1.py:
reuts#reuts-K53SD:~/python_examples$ cat ex1.py
#!/usr/bin/env python
from multiprocessing import Process
def f(name):
print 'hello', name
if __name__ == '__main__':
p = Process(target=f, args=('bob',))
p.start()
p.join()
reuts#reuts-K53SD:~/python_examples$ python ex1.py
hello bob
I had the issue that multiprocessing did not work on Spyder, and always landed here. I solved it by using threading instead of multiprocessing. as described here: https://pymotw.com/2/threading/
import threading
def worker(num):
"""thread worker function"""
print 'Worker: %s' % num
return
threads = []
for i in range(5):
t = threading.Thread(target=worker, args=(i,))
threads.append(t)
t.start()
Most likely your main process exits before sysout is flushed. Try this:
from multiprocessing import Process
import sys
def f(name):
print 'hello', name
if __name__ == '__main__':
p = Process(target=f, args=('bob',))
p.start()
p.join()
# make sure all output has been processed before we exit
sys.stdout.flush()
If this doesn't work, try adding time.sleep(1) as the last statement.
Try using this code (from standard manual). Works for me on windows. Another one did not work for me either :)
import multiprocessing as mp
def foo(q):
q.put('hello')
if __name__ == '__main__':
mp.set_start_method('spawn')
q = mp.Queue()
p = mp.Process(target=foo, args=(q,))
p.start()
print(q.get())
p.join()
For a Linux system, I am writing a program in Python, who spawns child processes. I am using the "multiprocessing" library and I am wondering if there is a method to call sub-processes with a different user than the current one. I'd like to be able to run each subprocess with a different user (like Postfix, for example.)
Any idea or pointers ?
modified python documentation example, I've added setuid in the function to be called, I'm not sure it fits to your needs and you may also need setgid, sedeuid setegid
from multiprocessing import Process
import os
def info(title):
print title
print 'module name:', __name__
print 'parent process:', os.getppid()
print 'process id:', os.getpid()
def f(name):
os.setuid(1000)
info('function f')
print 'hello', name
os.system('id')
if __name__ == '__main__':
info('main line')
p = Process(target=f, args=('bob',))
p.start()
p.join()
You could look in os.setpgid(pid, pgrp) direction.