Python Threads Aahz aahz@pobox.com http://starship.python.net/crew/aahz/ Powered by PythonPoint http://www.reportlab.com/ Meta Tutorial I'm hearing-impaired Please write questions if at all possible Take notes - or not Pop Quiz Slideshow on web Contents Goal: Use Threads! Thread Overview Python's Thread Library Two Applications Web Spider GUI Background Thread Part 1 What are threads? GIL Python threads Generic Threads Similar to processes Shared memory Light-weight Difficult to set up Especially cross-platform Why Use Threads? Efficiency/speed Responsiveness Algorithmic simplicity Python Threads Class-based Use threading, not thread Cross-platform Thread Library Python 1.5.2 vs. 2.0 Compile --with-thread Except on MS Windows and some Linux distributions Multi-CPU bug Creating/destroying large numbers of threads GIL Global Interpreter Lock (GIL) Full Documentation: http://www.python.org/doc/current/api/threads.html Only one Python thread can run GIL is your friend (really!) GIL in Action Which is faster? One Thread total = 1 for i in range(10000): total += 1 total = 1 for i in range(10000): total += 1 ,Two Threads, ,, ,total = 1,total = 1 ,for i in range(10000):,for i in range(10000): , total += 1, total += 1 Dealing with GIL sys.setcheckinterval() (default 10) C extensions can release GIL Blocking I/O releases GIL So does time.sleep(!=0) Multiple Processes Performance Tip python -O Also set PYTHONOPTIMIZE 15% performance boost Removes bytecodes (SET_LINENO) Fewer context switches! GIL and C Extensions Look for macros: Py_BEGIN_ALLOW_THREADS Py_END_ALLOW_THREADS Some common extensions: mxODBC - yes NumPy - no Share External Objects Files, GUI, DB connections Share External Objects Files, GUI, DB connections Don't Share External Objects Files, GUI, DB connections Don't Partial exception: print Still need to share? Use worker thread Create Python Threads Subclass threading.Thread Override __init__() and run() Do not override start() In __init__(), call Thread.__init__() Using Python Threads Instantiate thread object t = MyThread() Start the thread t.start() Methods/attribs from outside thread t.put('foo') if t.done: Non-threaded Example class Retriever: def __init__(self, URL): self.URL = URL self.page = self.getPage() retriever = Retriever('http://www.foo.com/') URLs = retriever.getLinks() Threaded Example from threading import Thread class Retriever(Thread): def __init__(self, URL): Thread.__init__(self) self.URL = URL def run(self): self.page = self.getPage() retriever = Retriever('http://www.foo.com/') retriever.start() while retriever.isAlive(): time.sleep(1) URLs = retriever.getLinks() Multiple Threads seeds = ['http://www.foo.com/', 'http://www.bar.com/', 'http://www.baz.com/'] threadList = [] URLs = [] for seed in Seed: retriever = Retriever(seed) retriever.start() threadList.append(retriever) for retriever in threadList: # join() is more efficient than sleep() retriever.join() URLs += retriever.getLinks() import Editorial How to import from threading import Thread, Semaphore or import threading Don't use from threading import * Thread Methods Module functions: activeCount() (not useful) enumerate() (not useful) Thread object methods: start() join() (somewhat useful) isAlive() (not useful) isDaemon() setDaemon() Unthreaded Spider SingleThreadSpider.py Compare Tools/webchecker/ Brute Thread Spider BruteThreadSpider.py Few changes from SingleThreadSpider.py Spawn one thread per retrieval Inefficient polling in main loop Part 2 Recap Thread Theory Python Thread Library Recap Part 1 GIL Creating threads Brute force threads Thread Order Non-determinate . Thread 1. Thread 2 .. .print "a,",.print "1,", .print "b,",.print "2,", .print "c,",.print "3,", Sample output 1, a, b, 2, c, 3, a, b, c, 1, 2, 3, 1, 2, 3, a, b, c, a, b, 1, 2, 3, c, Thread Communication Critical Section Lock Protects shared memory Only one thread accesses chunk of code aka "mutex", or "atomic operation" Wait/Notify Synchronizes actions between threads Threads wait for each other to finish a task More efficient than polling Thread Library Lock() RLock() Semaphore() Condition() Event() Queue.Queue() Lock() Basic building block Methods acquire(blocking) release() Critical Section Lock , Thread 1, Thread 2 ,, ,mutex.acquire(),... ,if myList:,... , work = myList.pop(),... ,mutex.release(),... ,...,mutex.acquire() ,...,if len(myList)<10: ,..., myList.append(work) ,...,mutex.release() GIL and Shared Vars Safe: one bytecode Single operations against Python basic types (e.g. appending to a list) Unsafe Multiple operations against Python variables (e.g. checking the length of a list before appending) or any operation that involves a callback to a class (e.g. the __getattr__ hook) GIL example Mutex only one thread , Thread 1, Thread 2 ,, ,myList.append(work),mutex.acquire() ,...,if myList: ,..., work = myList.pop() ,...,mutex.release() dis this disassemble source to byte codes Thread-unsafe statement If a single Python statement uses the same shared variable across multiple byte codes, or if there are multiple mutually-dependent shared variables, that statement is not thread-safe Misusing Lock() Lock() steps on itself mutex = Lock() mutex.acquire() ... mutex.acquire() # OOPS! Synch Two Threads class Synchronize: def __init__(self): self.lock = Lock() def wait(self): self.lock.acquire() self.lock.acquire() self.lock.release() def notify(self): self.lock.release() , Thread 1, Thread 2 ,, ,self.synch.wait(),... ,...,self.synch.notify() ,...,self.synch.wait() ,self.synch.notify(),... RLock() Mutex only Other threads cannot release RLock() Recursive Methods acquire(blocking) release() Using RLock() mutex = RLock() mutex.acquire() ... mutex.acquire() # Safe ... mutex.release() mutex.release() , Thread 1, Thread 2 ,, ,mutex.acquire(),... ,self.update(),... ,mutex.release(),... ,...,mutex.acquire() ,...,self.update() ,...,mutex.release() Semaphore() Restricts number of running threads In Python, primarily useful for simulations (but consider using microthreads) Methods Semaphore(value) acquire(blocking) release() Condition() Methods Condition(lock) acquire(blocking) release() wait(timeout) notify() notifyAll() Using Condition() Must use lock cond = Condition() cond.acquire() cond.wait() # or notify()/notifyAll() cond.release() Avoid timeout Creates polling loop, so inefficient Event() Thin wrapper for Condition() Don't have to mess with lock Only uses notifyAll(), so can be inefficient Methods set() clear() isSet() wait(timeout) TMTOWTDI Perl: There's More Than One Way To Do It Python: There should be one - and preferably only one - obvious way to do it Threads more like Perl Body factory Wheel factory Assembly Factory Objects 1 ,Body,Wheels ,, ,body.list,wheels.list ,body.rlock,wheels.rlock ,body.event,wheels.event ,assembly.event,assembly.event Assembly body.list body.rlock body.event wheels.list wheels.rlock wheels.event assembly.rlock assembly.event Queue() Does not use threading Can be used with thread Designed for subclassing Can implement stack, priority queue, etc. Simple! Handles both data protection and synchronization Queue() Objects Methods Queue(maxsize) put(item,block) get(block) qsize() empty() full() Raises exception when non-blocking Using Queue() ,Thread 1,Thread 2 ,, ,output = self.doWork(),... ,queue.put(output),... ,...,self.input = queue.get() ,...,output = self.doWork() ,...,queue.put(output) ,self.input = queue.get(),... Body factory Wheel factory Assembly Factory Objects 2 ,Body,Wheels ,, ,body.queue,wheels.queue Assembly body.queue wheels.queue assembly.rlock Body factory Wheel factory Packager Assembly Factory Objects 3 ,Body,Wheels ,, ,body.queue,wheels.queue Packager while 1: body = self.body.queue.get() wheels = self.wheels.queue.get() self.assembly.put( (body,wheels) ) Assembly assembly.queue Part 3 Recap Using Queues spider GUI (Tkinter) Recap Part 2 Data protection and synchronization Python Thread Library Queues are good Spider w/Queue ThreadPoolSpider.py Two queues Pass work to thread pool Get links back from thread pool Queue for both data and events Tkinter Intro This space intentionally left blank GUI building blocks Widgets Windows, buttons, checkboxes, text entry, listboxes Events Widget activation, keypress, mouse movement, mouse click, timers Widgets Geometry manager Register callbacks Events Event loop Trigger callbacks Tkinter resources Web http://www.python.org/topics/tkinter/doc.html Books Python and Tkinter Programming, John E. Grayson Fibonacci Fibonacci.py UI freezes during calc Frequent screen updates slow calc Threaded Fibonacci FibThreaded.py Tkinter needs to poll Use after event Single-element queue Use in non-blocking mode to minimize updates Must use "Quit" button Pop Quiz 1 How are threads and processes similar and different? What is the GIL? In what ways does the GIL make thread programming easier and harder? How do you create a thread in Python? What should not be shared between threads? What are "brute force" threads? Pop Quiz 2 Explain what each of the following is used for: Lock() RLock() Semaphore() Condition() Event() Queue.Queue() Why are queues great?