Python Multithreading/Multiprocessing Examples

--D. Thiebaut (talk) 15:27, 25 January 2017 (EST)

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Multithreading in Python

Warning! The multithreading module in Python does not provide true parallelism. Read up on the Global Interactive Lock if you want to learn more about this problem.

A good collection of examples can be found here: https://pymotw.com/2/threading/.

Example 1

# PythonThreading_Example1.py
# D. Thiebaut
# Taken and adapted from
# https://pymotw.com/2/threading/
#
# Python program illustrating how threads can
# be created and started in Python

from __future__ import print_function
import threading
import time


def worker( Id ):
    """
    thread worker function.  It receives an Id,
    and prints a message indicating it's starting.
    waits 0.5 seconds, prints another message, and dies.
    """
    print( 'Worker %d started\n' % Id )
    time.sleep( 0.5 )
    print( 'Worker %d finished\n' % Id )
    return

def main():
    """
    Spawns 5 threads that will run the worker function.
    Waits for the threads to finish, then stops.
    """
    threads = []
    for i in range(5):
        t = threading.Thread( target=worker, args=(i,) )
        threads.append(t)
        t.start()
    print( "Main has spawn all the threads" )

    # wait for all the threads to finish
    for t in threads:
        t.join()
        
    print( "Done!" )
    
main()

Computing Pi, Serial Version

# ComputingPy.py
# D. Thiebaut
#
# Computes an approximation of Pi by summing up
# a series of terms.  The more terms, the closer
# the approximation.
#

def f( x ):
    """
    The function being integrated, and which returns
    an approximation of Pi when summed up over an interval
    of integers.
    """
    return 4.0 / ( 1 + x*x )


def main():
    """
    Gets a number of terms from the user, then sums
    up the series and prints the resulting sum, which
    should approximate Pi.
    """

    # get the number of terms
    N = int( input( "> " ) )

    sum = 0.0          # where we sum up the individual
                       # intervals
    deltaX = 1.0 / N   # the interval

    # sum over N terms
    for i in range( N ):
        sum += f( i * deltaX )

    # display results
    print( "sum = %1.9f" % ( sum * deltaX ) )

main()

Computing Pi, Multithreaded Version

# ComputingPyMultiThreaded.py
# D. Thiebaut
#
# Computes an approximation of Pi by summing up
# a series of terms.  The more terms, the closer
# the approximation.
#
from __future__ import print_function
import threading


def f( x ):
    """
    The function being integrated, and which returns
    an approximation of Pi when summed up over an interval
    of integers.
    """
    return 4.0 / ( 1 + x*x )

class WorkerThread(threading.Thread):
    """
    That's the main worker thread, as a class.
    It gets initialized via __init__ with the interval
    and deltaX it sums up on.  It saves the accumulated
    sum in self.sum
    """
    
    def __init__( self, args ):
        """
        args must contain n1, n2, and deltaX, in that order.
        """
        threading.Thread.__init__( self, args=args )
        self.n1 = args[0]
        self.n2 = args[1]
        self.deltaX = args[2]

    def run( self ):
        """
        This will be called automatically by start() in
        main.  It's the method that does all the work.
        """
        self.sum = 0.0
        for i in range( self.n1, self.n2 ):
            self.sum += f( i * self.deltaX )

    def getSum( self ):
        """
        gets the computed sum.
        """
        return self.sum
    
def main():
    """
    Gets a number of terms from the user, then sums
    up the series and prints the resulting sum, which
    should approximate Pi.
    """

    # get the number of terms
    N = 1000000000 #int( input( "# steps > " ) )
    T = 1000       #int( input( "# threads? > " ) )
    
    deltaX = 1.0 / N    # the interval
    deltaN = int( N/T ) # the number of samples per thread

    # start T 
    n1 = 0
    threads = []
    for i in range( T ):
        n2 = n1 + deltaN
        t = WorkerThread( args=(n1, n2, deltaX,) )
        t.start()
        threads.append( t )
        n1 = n2

    # wait for the threads to be done
    for t in threads:
        t.join()

    # gather the sums
    sum = 0
    for t in threads:
        sum += t.getSum()

    # print result
    print( "sum = %1.9f" % ( sum * deltaX ) )

main()

Multiprocessing in Python

Note, these examples do not work in Idle. Instead, run them from the command line in a Terminal/Console window.

Example 1

# PythonMultiprocessing_Example1.py
# D. Thiebaut
# Taken and adapted from
# https://pymotw.com/2/threading/
#
# Python program illustrating how threads can
# be created and started in Python

from __future__ import print_function
import multiprocessing
import time


def worker( Id ):
    """
    Process worker function.  It receives an Id,
    and prints a message indicating it's starting.
    waits 0.5 seconds, prints another message, and dies.
    """
    print( 'Worker %d started\n' % Id )
    time.sleep( 2.5 )
    print( 'Worker %d finished\n' % Id )
    return

def main():
    """
    Spawns 5 processes that will run the worker function.
    Waits for the threads to finish, then stops.
    """
    jobs = []
    for i in range(5):
        p = multiprocessing.Process( target=worker, args=(i,) )
        jobs.append(p)
        p.start()
        
    print( "Main has spawn all the threads" )

    # wait for all the threads to finish
    for p in jobs:
        p.join()
        
    print( "Done!" )
    
main()