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def consumer(): while True: cvacquire() while not item_is_available(): cvwait() # Wait for an item to show up cvrelease() consume_item()
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A subtle aspect of using condition variables is that if there are multiple threads waiting on the same condition, the notify() operation may awaken one or more of them (this behavior often depends on the underlying operating system) Because of this, there is always a possibility that a thread will awaken only to find that the condition of interest no longer holdsThis explains, for instance, why a while loop is used in the consumer() function If the thread awakens, but the produced item is already gone, it just goes back to waiting for the next signal
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Great care must be taken when working with any of the locking primitives such as Lock, RLock, or Semaphore Mismanagement of locks is a frequent source of deadlock or race conditions Code that relies on a lock should always make sure locks get properly released even when exceptions occurTypical code looks like this:
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try: lockacquire() # critical section statements finally: lockrelease()
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Alternatively, all of the locks also support the context management protocol which is a little cleaner:
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with lock: # critical section statements
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In this last example, the lock is automatically acquired by the with statement and released when control flow leaves the context Also, as a general rule you should avoid writing code where more than one lock is acquired at any given time For example:
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with lock_A: # critical section statements with lock_B: # critical section on B statements
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This is usually a good way to have your application mysteriously deadlock Although there are strategies for avoiding this (for example, hierarchical locking), you re often better off writing code that avoids this altogether
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Threads do not have any methods for forceful termination or suspensionThis omission is by design and due to the intrinsic complexity of writing threaded programs For example, if a thread has acquired a lock, forcefully terminating or suspending it before it is able to release the lock may cause the entire application to deadlock Moreover, it is generally not possible to simply release all locks on termination either because complicated thread synchronization often involves locking and unlocking operations that must be carried out in a very precise sequence to work If you want to support termination or suspension, you need to build these features yourselfTypically, it s done by making a thread run in a loop that periodically checks its status to see if it should terminate For example:
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class StoppableThread(threadingThread): def _ _init_ _(self): threadingThread_ _init_ _() self_terminate = False self_suspend_lock = threadingLock() def terminate(self): self_terminate = True def suspend(self): self_suspend_lockacquire() def resume(self): self_suspend_lockrelease() def run(self): while True: if self_terminate: break self_suspend_lockacquire() self_suspend_lockrelease() statements
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Keep in mind that to make this approach work reliability, the thread should take great care not to perform any kind of blocking I/O operation For example, if the thread blocks waiting for data to arrive, it won t terminate until it wakes up from that operation Because of this, you would probably want to make the implementation use timeouts, non-blocking I/O, and other advanced features to make sure that that the termination check executes every so often
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The following utility functions are available:
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active_count()
Returns the number of currently active Thread objects
current_thread()
Returns the Thread object corresponding to the caller s thread of control
enumerate()
Returns a list of all currently active Thread objects
local()
Returns a local object that allows for the storage of thread-local dataThis object is guaranteed to be unique in each thread
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setprofile(func)
Sets a profile function that will be used for all threads created func is passed to syssetprofile() before each thread starts running
settrace(func)
Sets a tracing function that will be used for all threads created func is passed to syssettrace() before each thread starts running
stack_size([size])
Returns the stack size used when creating new threads If an optional integer size is given, it sets the stack size to be used for creating new threads size can be a value that is 32768 (32KB) or greater and a multiple of 4096 (4KB) for maximum portability A ThreadError exception is raised if this operation isn t supported on the system