= [wiki: Python parallèle]
PageOutline
#!comment
A bosser
import concurrent.futures
https://pythonhosted.org/joblib/parallel.html
http://www.tutorialspoint.com/python/python_multithreading.htm
https://bitbucket.org/stackless-dev/stackless/wiki/Home
== Starting a New Thread
To spawn another thread, you need to call following method available in
thread module:
#!python
thread.start_new_thread ( function, args[, kwargs] )
This method call enables a fast and efficient way to create new threads in both Linux
and Windows.
The method call returns immediately and the child thread starts and calls function
with the passed list of agrs. When function returns, the thread terminates.
Here, args is a tuple of arguments; use an empty tuple to call function without
passing any arguments. kwargs is an optional dictionary of keyword arguments.
#!python
import thread
import time
# Define a function for the thread
def print_time( threadName, delay):
count = 0
while count < 5:
time.sleep(delay)
count += 1
print "%s: %s" % ( threadName, time.ctime(time.time()) )
# Create two threads as follows
try:
thread.start_new_thread( print_time, ("Thread-1", 2, ) )
thread.start_new_thread( print_time, ("Thread-2", 4, ) )
except:
print "Error: unable to start thread"
while 1:
pass
When the above code is executed, it produces the following result −
Thread-1: Thu Jan 22 15:42:17 2009
Thread-1: Thu Jan 22 15:42:19 2009
Thread-2: Thu Jan 22 15:42:19 2009
Thread-1: Thu Jan 22 15:42:21 2009
Thread-2: Thu Jan 22 15:42:23 2009
Thread-1: Thu Jan 22 15:42:23 2009
Thread-1: Thu Jan 22 15:42:25 2009
Thread-2: Thu Jan 22 15:42:27 2009
Thread-2: Thu Jan 22 15:42:31 2009
Thread-2: Thu Jan 22 15:42:35 2009Although it is very effective for low-level threading, but the thread module is
very limited compared to the newer threading module.
== The Threading Module
The newer threading module included with Python 2.4 provides much more powerful,
high-level support for threads than the thread module discussed in the previous section.
The threading module exposes all the methods of the thread module and provides
some additional methods:
- threading.activeCount(): Returns the number of thread objects that are active.
- threading.currentThread(): Returns the number of thread objects in the caller’s thread control.
- threading.enumerate(): Returns a list of all thread objects that are currently active.
In addition to the methods, the threading module has the Thread class that implements
threading. The methods provided by the Thread class are as follows:
- run(): The run() method is the entry point for a thread.
- start(): The start() method starts a thread by calling the run method.
- join([time]): The join() waits for threads to terminate.
- isAlive(): The isAlive() method checks whether a thread is still executing.
- getName(): The getName() method returns the name of a thread.
- setName(): The setName() method sets the name of a thread.
== Creating Thread Using Threading Module
To implement a new thread using the threading module, you have to do the following −
- Define a new subclass of the Thread class.
- Override the init(self [,args]) method to add additional arguments.
- Then, override the run(self [,args]) method to implement what the thread should
do when started.
Once you have created the new Thread subclass, you can create an instance of it and
then start a new thread by invoking the start(), which in turn calls run() method.
#!python
import threading
import time
class myThread (threading.Thread):
def init(self, threadID, name, delay):
threading.Thread.init(self)
self.threadID = threadID
self.name = name
self.delay = delay
def run(self):
print "Starting " + self.name
print_time(self.name, self.delay, 5)
print "Exiting " + self.name
def print_time(threadName, delay, counter):
while counter:
time.sleep(delay)
print "%s: %s" % (threadName, time.ctime(time.time()))
counter -= 1
# Create new threads
thread1 = myThread(1, "Thread-1", 1)
thread2 = myThread(2, "Thread-2", 2)
# Start new Threads
thread1.start()
thread2.start()
print "Exiting Main Thread"
When the above code is executed, it produces the following result:
Starting Thread-1
Starting Thread-2
Exiting Main Thread
Thread-1: Thu Mar 21 09:10:03 2013
Thread-1: Thu Mar 21 09:10:04 2013
Thread-2: Thu Mar 21 09:10:04 2013
Thread-1: Thu Mar 21 09:10:05 2013
Thread-1: Thu Mar 21 09:10:06 2013
Thread-2: Thu Mar 21 09:10:06 2013
Thread-1: Thu Mar 21 09:10:07 2013
Exiting Thread-1
Thread-2: Thu Mar 21 09:10:08 2013
Thread-2: Thu Mar 21 09:10:10 2013
Thread-2: Thu Mar 21 09:10:12 2013
Exiting Thread-2== Synchronizing Threads
The threading module provided with Python includes a simple-to-implement locking
mechanism that allows you to synchronize threads. A new lock is created by calling
the Lock() method, which returns the new lock.
The acquire(blocking) method of the new lock object is used to force threads to
run synchronously. The optional blocking parameter enables you to control whether
the thread waits to acquire the lock.
If blocking is set to 0, the thread returns immediately with a 0 value if the lock
cannot be acquired and with a 1 if the lock was acquired. If blocking is set to 1,
the thread blocks and wait for the lock to be released.
The release() method of the new lock object is used to release the lock when it
is no longer required.
#!python
import threading
import time
class myThread (threading.Thread):
def init(self, threadID, name, counter):
threading.Thread.init(self)
self.threadID = threadID
self.name = name
self.counter = counter
def run(self):
print "Starting " + self.name
# Get lock to synchronize threads
threadLock.acquire()
print_time(self.name, self.counter, 3)
# Free lock to release next thread
threadLock.release()
def print_time(threadName, delay, counter):
while counter:
time.sleep(delay)
print "%s: %s" % (threadName, time.ctime(time.time()))
counter -= 1
threadLock = threading.Lock()
threads = []
# Create new threads
thread1 = myThread(1, "Thread-1", 1)
thread2 = myThread(2, "Thread-2", 2)
# Start new Threads
thread1.start()
thread2.start()
# Add threads to thread list
threads.append(thread1)
threads.append(thread2)
# Wait for all threads to complete
for t in threads:
t.join()
print "Exiting Main Thread"
When the above code is executed, it produces the following result:
Starting Thread-1
Starting Thread-2
Thread-1: Thu Mar 21 09:11:28 2013
Thread-1: Thu Mar 21 09:11:29 2013
Thread-1: Thu Mar 21 09:11:30 2013
Thread-2: Thu Mar 21 09:11:32 2013
Thread-2: Thu Mar 21 09:11:34 2013
Thread-2: Thu Mar 21 09:11:36 2013
Exiting Main Thread== Multithreaded Priority Queue
The Queue module allows you to create a new queue object that can hold a specific
number of items. There are following methods to control the Queue:
- get(): The get() removes and returns an item from the queue.
- put(): The put adds item to a queue.
- qsize() : The qsize() returns the number of items that are currently in the queue.
- empty(): The empty( ) returns True if queue is empty; otherwise, False.
- full(): the full() returns True if queue is full; otherwise, False.
#!python
import Queue
import threading
import time
exitFlag = 0
class myThread(threading.Thread):
def init(self, threadID, name, q):
threading.Thread.init(self)
self.threadID = threadID
self.name = name
self.q = q
def run(self):
print "Starting " + self.name
process_data(self.name, self.q)
print "Exiting " + self.name
def process_data(threadName, q):
while not exitFlag:
queueLock.acquire()
if not workQueue.empty():
data = q.get()
queueLock.release()
print "%s processing %s" % (threadName, data)
else:
queueLock.release()
time.sleep(1)
threadList = ["Thread-1", "Thread-2", "Thread-3"]
nameList = ["One", "Two", "Three", "Four", "Five"]
queueLock = threading.Lock()
workQueue = Queue.Queue(10)
threads = []
threadID = 1
# Create new threads
for tName in threadList:
thread = myThread(threadID, tName, workQueue)
thread.start()
threads.append(thread)
threadID += 1
# Fill the queue
queueLock.acquire()
for word in nameList:
workQueue.put(word)
queueLock.release()
# Wait for queue to empty
while not workQueue.empty():
pass
# Notify threads it's time to exit
exitFlag = 1
# Wait for all threads to complete
for t in threads:
t.join()
print "Exiting Main Thread"
When the above code is executed, it produces the following result −
Starting Thread-1
Starting Thread-2
Starting Thread-3
Thread-1 processing One
Thread-2 processing Two
Thread-3 processing Three
Thread-1 processing Four
Thread-2 processing Five
Exiting Thread-3
Exiting Thread-1
Exiting Thread-2
Exiting Main Thread