17.1. threading — 基于线程的并行 ¶

17.1.1. 线程本地数据 ¶

Thread-local data is data whose values are thread specific. To manage thread-local data, just create an instance of local (or a subclass) and store attributes on it:

mydata = threading.local()
mydata.x = 1
								

实例的值异于单独线程。

class threading. local ¶

表示线程本地数据的类。

For more details and extensive examples, see the documentation string of the _threading_local 模块。

17.1.2. 线程对象 ¶

Thread class represents an activity that is run in a separate thread of control. There are two ways to specify the activity: by passing a callable object to the constructor, or by overriding the run() method in a subclass. No other methods (except for the constructor) should be overridden in a subclass. In other words, only override the __init__() and run() methods of this class.

Once a thread object is created, its activity must be started by calling the thread’s start() method. This invokes the run() method in a separate thread of control.

Once the thread’s activity is started, the thread is considered ‘alive’. It stops being alive when its run() method terminates – either normally, or by raising an unhandled exception. The is_alive() method tests whether the thread is alive.

其它线程可以调用线程的 join() method. This blocks the calling thread until the thread whose join() method is called is terminated.

A thread has a name. The name can be passed to the constructor, and read or changed through the name 属性。

A thread can be flagged as a “daemon thread”. The significance of this flag is that the entire Python program exits when only daemon threads are left. The initial value is inherited from the creating thread. The flag can be set through the daemon property or the daemon constructor argument.

There is a “main thread” object; this corresponds to the initial thread of control in the Python program. It is not a daemon thread.

There is the possibility that “dummy thread objects” are created. These are thread objects corresponding to “alien threads”, which are threads of control started outside the threading module, such as directly from C code. Dummy thread objects have limited functionality; they are always considered alive and daemonic, and cannot be join() ed. They are never deleted, since it is impossible to detect the termination of alien threads.

class threading. Thread ( group=None , target=None , name=None , args=() , kwargs={} , * , daemon=None ) ¶

始终应采用关键词自变量调用此构造函数。自变量：

group 应该为 None ；预留以供未来扩展当 ThreadGroup 类被实现。

target is the callable object to be invoked by the run() method. Defaults to None ，意味着什么都不调用。

name is the thread name. By default, a unique name is constructed of the form “Thread- N ” where N is a small decimal number.

args is the argument tuple for the target invocation. Defaults to () .

kwargs is a dictionary of keyword arguments for the target invocation. Defaults to {} .

If not None , daemon explicitly sets whether the thread is daemonic. If None (the default), the daemonic property is inherited from the current thread.

若子类覆盖构造函数，它必须确保援引基类构造函数 ( Thread.__init__() ) 在对其它线程做任何事情之前。

3.3 版改变： 添加 daemon 自变量。

start ( ) ¶

启动线程活动。

It must be called at most once per thread object. It arranges for the object’s run() method to be invoked in a separate thread of control.

此方法将引发 RuntimeError if called more than once on the same thread object.

run ( ) ¶

表示线程活动的方法。

You may override this method in a subclass. The standard run() method invokes the callable object passed to the object’s constructor as the target argument, if any, with sequential and keyword arguments taken from the args and kwargs arguments, respectively.

join ( timeout=None ) ¶

Wait until the thread terminates. This blocks the calling thread until the thread whose join() method is called terminates – either normally or through an unhandled exception – or until the optional timeout occurs.

当 timeout argument is present and not None , it should be a floating point number specifying a timeout for the operation in seconds (or fractions thereof). As join() always returns None , you must call is_alive() after join() to decide whether a timeout happened – if the thread is still alive, the join() call timed out.

当 timeout argument is not present or None , the operation will block until the thread terminates.

A thread can be join() ed many times.

join() 引发 RuntimeError if an attempt is made to join the current thread as that would cause a deadlock. It is also an error to join() a thread before it has been started and attempts to do so raise the same exception.

name ¶

A string used for identification purposes only. It has no semantics. Multiple threads may be given the same name. The initial name is set by the constructor.

getName ( ) ¶

setName ( ) ¶

旧的 Getter/Setter API 为 name ；直接将其用作代替特性。

ident ¶

The ‘thread identifier’ of this thread or None if the thread has not been started. This is a nonzero integer. See the get_ident() function. Thread identifiers may be recycled when a thread exits and another thread is created. The identifier is available even after the thread has exited.

is_alive ( ) ¶

返回线程是否存活。

This method returns True just before the run() method starts until just after the run() method terminates. The module function enumerate() returns a list of all alive threads.

daemon ¶

A boolean value indicating whether this thread is a daemon thread (True) or not (False). This must be set before start() is called, otherwise RuntimeError is raised. Its initial value is inherited from the creating thread; the main thread is not a daemon thread and therefore all threads created in the main thread default to daemon = False .

The entire Python program exits when no alive non-daemon threads are left.

isDaemon ( ) ¶

setDaemon ( ) ¶

旧的 Getter/Setter API 为 daemon ；直接将其用作代替特性。

17.1.3. 锁对象 ¶

A primitive lock is a synchronization primitive that is not owned by a particular thread when locked. In Python, it is currently the lowest level synchronization primitive available, implemented directly by the _thread extension module.

A primitive lock is in one of two states, “locked” or “unlocked”. It is created in the unlocked state. It has two basic methods, acquire() and release() . When the state is unlocked, acquire() changes the state to locked and returns immediately. When the state is locked, acquire() blocks until a call to release() in another thread changes it to unlocked, then the acquire() call resets it to locked and returns. The release() method should only be called in the locked state; it changes the state to unlocked and returns immediately. If an attempt is made to release an unlocked lock, a RuntimeError 会被引发。

锁还支持 上下文管理协议 .

When more than one thread is blocked in acquire() waiting for the state to turn to unlocked, only one thread proceeds when a release() call resets the state to unlocked; which one of the waiting threads proceeds is not defined, and may vary across implementations.

All methods are executed atomically.

class threading. Lock ¶

The class implementing primitive lock objects. Once a thread has acquired a lock, subsequent attempts to acquire it block, until it is released; any thread may release it.

注意， Lock is actually a factory function which returns an instance of the most efficient version of the concrete Lock class that is supported by the platform.

acquire ( blocking=True , timeout=-1 ) ¶

获得锁，阻塞或非阻塞。

When invoked with the blocking argument set to True (the default), block until the lock is unlocked, then set it to locked and return True .

When invoked with the blocking argument set to False , do not block. If a call with blocking 设为 True would block, return False immediately; otherwise, set the lock to locked and return True .

When invoked with the floating-point timeout argument set to a positive value, block for at most the number of seconds specified by timeout and as long as the lock cannot be acquired. A timeout argument of -1 specifies an unbounded wait. It is forbidden to specify a timeout when blocking 为 False。

返回值为 True if the lock is acquired successfully, False if not (for example if the timeout expired).

3.2 版改变： timeout parameter is new.

3.2 版改变： Lock acquisition can now be interrupted by signals on POSIX if the underlying threading implementation supports it.

release ( ) ¶

Release a lock. This can be called from any thread, not only the thread which has acquired the lock.

When the lock is locked, reset it to unlocked, and return. If any other threads are blocked waiting for the lock to become unlocked, allow exactly one of them to proceed.

When invoked on an unlocked lock, a RuntimeError 被引发。

没有返回值。

17.1.4. RLock 对象 ¶

A reentrant lock is a synchronization primitive that may be acquired multiple times by the same thread. Internally, it uses the concepts of “owning thread” and “recursion level” in addition to the locked/unlocked state used by primitive locks. In the locked state, some thread owns the lock; in the unlocked state, no thread owns it.

To lock the lock, a thread calls its acquire() method; this returns once the thread owns the lock. To unlock the lock, a thread calls its release() 方法。 acquire() / release() call pairs may be nested; only the final release() ( release() of the outermost pair) resets the lock to unlocked and allows another thread blocked in acquire() to proceed.

Reentrant locks also support the 上下文管理协议 .

class threading. RLock ¶

This class implements reentrant lock objects. A reentrant lock must be released by the thread that acquired it. Once a thread has acquired a reentrant lock, the same thread may acquire it again without blocking; the thread must release it once for each time it has acquired it.

注意， RLock is actually a factory function which returns an instance of the most efficient version of the concrete RLock class that is supported by the platform.

acquire ( blocking=True , timeout=-1 ) ¶

获得锁，阻塞或非阻塞。

When invoked without arguments: if this thread already owns the lock, increment the recursion level by one, and return immediately. Otherwise, if another thread owns the lock, block until the lock is unlocked. Once the lock is unlocked (not owned by any thread), then grab ownership, set the recursion level to one, and return. If more than one thread is blocked waiting until the lock is unlocked, only one at a time will be able to grab ownership of the lock. There is no return value in this case.

When invoked with the blocking argument set to true, do the same thing as when called without arguments, and return true.

When invoked with the blocking argument set to false, do not block. If a call without an argument would block, return false immediately; otherwise, do the same thing as when called without arguments, and return true.

When invoked with the floating-point timeout argument set to a positive value, block for at most the number of seconds specified by timeout and as long as the lock cannot be acquired. Return true if the lock has been acquired, false if the timeout has elapsed.

3.2 版改变： timeout parameter is new.

release ( ) ¶

Release a lock, decrementing the recursion level. If after the decrement it is zero, reset the lock to unlocked (not owned by any thread), and if any other threads are blocked waiting for the lock to become unlocked, allow exactly one of them to proceed. If after the decrement the recursion level is still nonzero, the lock remains locked and owned by the calling thread.

Only call this method when the calling thread owns the lock. A RuntimeError is raised if this method is called when the lock is unlocked.

没有返回值。

17.1.5. 条件对象 ¶

A condition variable is always associated with some kind of lock; this can be passed in or one will be created by default. Passing one in is useful when several condition variables must share the same lock. The lock is part of the condition object: you don’t have to track it separately.

条件变量服从 上下文管理协议 : using the with statement acquires the associated lock for the duration of the enclosed block. The acquire() and release() methods also call the corresponding methods of the associated lock.

Other methods must be called with the associated lock held. The wait() method releases the lock, and then blocks until another thread awakens it by calling notify() or notify_all() . Once awakened, wait() re-acquires the lock and returns. It is also possible to specify a timeout.

notify() method wakes up one of the threads waiting for the condition variable, if any are waiting. The notify_all() method wakes up all threads waiting for the condition variable.

Note: the notify() and notify_all() methods don’t release the lock; this means that the thread or threads awakened will not return from their wait() call immediately, but only when the thread that called notify() or notify_all() finally relinquishes ownership of the lock.

The typical programming style using condition variables uses the lock to synchronize access to some shared state; threads that are interested in a particular change of state call wait() repeatedly until they see the desired state, while threads that modify the state call notify() or notify_all() when they change the state in such a way that it could possibly be a desired state for one of the waiters. For example, the following code is a generic producer-consumer situation with unlimited buffer capacity:

# Consume one item
with cv:
    while not an_item_is_available():
        cv.wait()
    get_an_available_item()
# Produce one item
with cv:
    make_an_item_available()
    cv.notify()
								

while loop checking for the application’s condition is necessary because wait() can return after an arbitrary long time, and the condition which prompted the notify() call may no longer hold true. This is inherent to multi-threaded programming. The wait_for() method can be used to automate the condition checking, and eases the computation of timeouts:

# Consume an item
with cv:
    cv.wait_for(an_item_is_available)
    get_an_available_item()
								

To choose between notify() and notify_all() , consider whether one state change can be interesting for only one or several waiting threads. E.g. in a typical producer-consumer situation, adding one item to the buffer only needs to wake up one consumer thread.

class threading. Condition ( lock=None ) ¶

This class implements condition variable objects. A condition variable allows one or more threads to wait until they are notified by another thread.

若 lock argument is given and not None , it must be a Lock or RLock object, and it is used as the underlying lock. Otherwise, a new RLock object is created and used as the underlying lock.

3.3 版改变： changed from a factory function to a class.

acquire ( *args ) ¶

Acquire the underlying lock. This method calls the corresponding method on the underlying lock; the return value is whatever that method returns.

release ( ) ¶

Release the underlying lock. This method calls the corresponding method on the underlying lock; there is no return value.

wait ( timeout=None ) ¶

Wait until notified or until a timeout occurs. If the calling thread has not acquired the lock when this method is called, a RuntimeError 被引发。

This method releases the underlying lock, and then blocks until it is awakened by a notify() or notify_all() call for the same condition variable in another thread, or until the optional timeout occurs. Once awakened or timed out, it re-acquires the lock and returns.

当 timeout argument is present and not None , it should be a floating point number specifying a timeout for the operation in seconds (or fractions thereof).

When the underlying lock is an RLock , it is not released using its release() method, since this may not actually unlock the lock when it was acquired multiple times recursively. Instead, an internal interface of the RLock class is used, which really unlocks it even when it has been recursively acquired several times. Another internal interface is then used to restore the recursion level when the lock is reacquired.

返回值为 True 除非给定 timeout 过期，在这种情况下为 False .

3.2 版改变： 以前，方法总是返回 None .

wait_for ( predicate , timeout=None ) ¶

Wait until a condition evaluates to true. predicate should be a callable which result will be interpreted as a boolean value. A timeout may be provided giving the maximum time to wait.

This utility method may call wait() repeatedly until the predicate is satisfied, or until a timeout occurs. The return value is the last return value of the predicate and will evaluate to False if the method timed out.

Ignoring the timeout feature, calling this method is roughly equivalent to writing:

while not predicate():
    cv.wait()
												

Therefore, the same rules apply as with wait() : The lock must be held when called and is re-acquired on return. The predicate is evaluated with the lock held.

3.2 版新增。

notify ( n=1 ) ¶

By default, wake up one thread waiting on this condition, if any. If the calling thread has not acquired the lock when this method is called, a RuntimeError 被引发。

This method wakes up at most n of the threads waiting for the condition variable; it is a no-op if no threads are waiting.

The current implementation wakes up exactly n threads, if at least n threads are waiting. However, it’s not safe to rely on this behavior. A future, optimized implementation may occasionally wake up more than n threads.

Note: an awakened thread does not actually return from its wait() call until it can reacquire the lock. Since notify() does not release the lock, its caller should.

notify_all ( ) ¶

Wake up all threads waiting on this condition. This method acts like notify() , but wakes up all waiting threads instead of one. If the calling thread has not acquired the lock when this method is called, a RuntimeError 被引发。

17.1.6. 信号量对象 ¶

This is one of the oldest synchronization primitives in the history of computer science, invented by the early Dutch computer scientist Edsger W. Dijkstra (he used the names P() and V() 而不是 acquire() and release() ).

A semaphore manages an internal counter which is decremented by each acquire() call and incremented by each release() call. The counter can never go below zero; when acquire() finds that it is zero, it blocks, waiting until some other thread calls release() .

信号量还支持 上下文管理协议 .

class threading. Semaphore ( value=1 ) ¶

This class implements semaphore objects. A semaphore manages an atomic counter representing the number of release() calls minus the number of acquire() calls, plus an initial value. The acquire() method blocks if necessary until it can return without making the counter negative. If not given, value defaults to 1.

The optional argument gives the initial value for the internal counter; it defaults to 1 。若 value given is less than 0, ValueError 被引发。

3.3 版改变： changed from a factory function to a class.

acquire ( blocking=True , timeout=None ) ¶

Acquire a semaphore.

When invoked without arguments:

• If the internal counter is larger than zero on entry, decrement it by one and return true immediately.
• If the internal counter is zero on entry, block until awoken by a call to release() . Once awoken (and the counter is greater than 0), decrement the counter by 1 and return true. Exactly one thread will be awoken by each call to release() . The order in which threads are awoken should not be relied on.

When invoked with blocking set to false, do not block. If a call without an argument would block, return false immediately; otherwise, do the same thing as when called without arguments, and return true.

When invoked with a timeout other than None , it will block for at most timeout seconds. If acquire does not complete successfully in that interval, return false. Return true otherwise.

3.2 版改变： timeout parameter is new.

release ( ) ¶

Release a semaphore, incrementing the internal counter by one. When it was zero on entry and another thread is waiting for it to become larger than zero again, wake up that thread.

class threading. BoundedSemaphore ( value=1 ) ¶

Class implementing bounded semaphore objects. A bounded semaphore checks to make sure its current value doesn’t exceed its initial value. If it does, ValueError is raised. In most situations semaphores are used to guard resources with limited capacity. If the semaphore is released too many times it’s a sign of a bug. If not given, value defaults to 1.

3.3 版改变： changed from a factory function to a class.

maxconnections = 5
# ...
pool_sema = BoundedSemaphore(value=maxconnections)
									

with pool_sema:
    conn = connectdb()
    try:
        # ... use connection ...
    finally:
        conn.close()
									

17.1.7. 事件对象 ¶

This is one of the simplest mechanisms for communication between threads: one thread signals an event and other threads wait for it.

An event object manages an internal flag that can be set to true with the set() method and reset to false with the clear() method. The wait() method blocks until the flag is true.

class threading. Event ¶

Class implementing event objects. An event manages a flag that can be set to true with the set() method and reset to false with the clear() method. The wait() method blocks until the flag is true. The flag is initially false.

3.3 版改变： changed from a factory function to a class.

is_set ( ) ¶

Return true if and only if the internal flag is true.

set ( ) ¶

Set the internal flag to true. All threads waiting for it to become true are awakened. Threads that call wait() once the flag is true will not block at all.

clear ( ) ¶

Reset the internal flag to false. Subsequently, threads calling wait() will block until set() is called to set the internal flag to true again.

wait ( timeout=None ) ¶

Block until the internal flag is true. If the internal flag is true on entry, return immediately. Otherwise, block until another thread calls set() to set the flag to true, or until the optional timeout occurs.

When the timeout argument is present and not None , it should be a floating point number specifying a timeout for the operation in seconds (or fractions thereof).

This method returns true if and only if the internal flag has been set to true, either before the wait call or after the wait starts, so it will always return True except if a timeout is given and the operation times out.

3.1 版改变： 以前，方法总是返回 None .

17.1.8. 计时器对象 ¶

This class represents an action that should be run only after a certain amount of time has passed — a timer. Timer 是子类对于 Thread and as such also functions as an example of creating custom threads.

Timers are started, as with threads, by calling their start() method. The timer can be stopped (before its action has begun) by calling the cancel() method. The interval the timer will wait before executing its action may not be exactly the same as the interval specified by the user.

例如：

def hello():
    print("hello, world")
t = Timer(30.0, hello)
t.start()  # after 30 seconds, "hello, world" will be printed
								

class threading. Timer ( interval , function , args=None , kwargs=None ) ¶

Create a timer that will run function 采用自变量 args 和关键词自变量 kwargs , after interval seconds have passed. If args is None (the default) then an empty list will be used. If kwargs is None (the default) then an empty dict will be used.

3.3 版改变： changed from a factory function to a class.

cancel ( ) ¶

Stop the timer, and cancel the execution of the timer’s action. This will only work if the timer is still in its waiting stage.

17.1.9. 屏障对象 ¶

This class provides a simple synchronization primitive for use by a fixed number of threads that need to wait for each other. Each of the threads tries to pass the barrier by calling the wait() method and will block until all of the threads have made their wait() calls. At this point, the threads are released simultaneously.

The barrier can be reused any number of times for the same number of threads.

As an example, here is a simple way to synchronize a client and server thread:

b = Barrier(2, timeout=5)
def server():
    start_server()
    b.wait()
    while True:
        connection = accept_connection()
        process_server_connection(connection)
def client():
    b.wait()
    while True:
        connection = make_connection()
        process_client_connection(connection)
								

class threading. Barrier ( parties , action=None , timeout=None ) ¶

Create a barrier object for parties number of threads. An action , when provided, is a callable to be called by one of the threads when they are released. timeout is the default timeout value if none is specified for the wait() 方法。

wait ( timeout=None ) ¶

Pass the barrier. When all the threads party to the barrier have called this function, they are all released simultaneously. If a timeout is provided, it is used in preference to any that was supplied to the class constructor.

The return value is an integer in the range 0 to parties – 1, different for each thread. This can be used to select a thread to do some special housekeeping, e.g.:

i = barrier.wait()
if i == 0:
    # Only one thread needs to print this
    print("passed the barrier")
												

If an action was provided to the constructor, one of the threads will have called it prior to being released. Should this call raise an error, the barrier is put into the broken state.

If the call times out, the barrier is put into the broken state.

This method may raise a BrokenBarrierError exception if the barrier is broken or reset while a thread is waiting.

reset ( ) ¶

Return the barrier to the default, empty state. Any threads waiting on it will receive the BrokenBarrierError 异常。

Note that using this function may can require some external synchronization if there are other threads whose state is unknown. If a barrier is broken it may be better to just leave it and create a new one.

abort ( ) ¶

Put the barrier into a broken state. This causes any active or future calls to wait() to fail with the BrokenBarrierError . Use this for example if one of the needs to abort, to avoid deadlocking the application.

It may be preferable to simply create the barrier with a sensible timeout value to automatically guard against one of the threads going awry.

parties ¶

The number of threads required to pass the barrier.

n_waiting ¶

The number of threads currently waiting in the barrier.

broken ¶

A boolean that is True if the barrier is in the broken state.

exception threading. BrokenBarrierError ¶

This exception, a subclass of RuntimeError , is raised when the Barrier object is reset or broken.

17.1.10. 使用锁、条件和信号量在 with 语句 ¶

All of the objects provided by this module that have acquire() and release() methods can be used as context managers for a with statement. The acquire() method will be called when the block is entered, and release() will be called when the block is exited. Hence, the following snippet:

with some_lock:
    # do something...
								

相当于：

some_lock.acquire()
try:
    # do something...
finally:
    some_lock.release()
								

目前， Lock , RLock , Condition , Semaphore ，和 BoundedSemaphore objects may be used as with statement context managers.