
``threading`` --- Thread-based parallelism
******************************************

**Source code:** Lib/threading.py

======================================================================

This module constructs higher-level threading interfaces on top of the
lower level ``_thread`` module.  See also the ``queue`` module.

The ``dummy_threading`` module is provided for situations where
``threading`` cannot be used because ``_thread`` is missing.

Note: While they are not listed below, the ``camelCase`` names used for
  some methods and functions in this module in the Python 2.x series
  are still supported by this module.

**CPython implementation detail:** Due to the *Global Interpreter
Lock*, in CPython only one thread can execute Python code at once
(even though certain performance-oriented libraries might overcome
this limitation). If you want your application to make better of use
of the computational resources of multi-core machines, you are advised
to use ``multiprocessing`` or
``concurrent.futures.ProcessPoolExecutor``. However, threading is
still an appropriate model if you want to run multiple I/O-bound tasks
simultaneously.

This module defines the following functions and objects:

threading.active_count()

   Return the number of ``Thread`` objects currently alive.  The
   returned count is equal to the length of the list returned by
   ``enumerate()``.

threading.Condition()

   A factory function that returns a new condition variable object. A
   condition variable allows one or more threads to wait until they
   are notified by another thread.

   See *Condition Objects*.

threading.current_thread()

   Return the current ``Thread`` object, corresponding to the caller's
   thread of control.  If the caller's thread of control was not
   created through the ``threading`` module, a dummy thread object
   with limited functionality is returned.

threading.enumerate()

   Return a list of all ``Thread`` objects currently alive.  The list
   includes daemonic threads, dummy thread objects created by
   ``current_thread()``, and the main thread.  It excludes terminated
   threads and threads that have not yet been started.

threading.Event()

   A factory function that returns a new event object.  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.

   See *Event Objects*.

class class threading.local

   A class that represents thread-local data.  Thread-local data are
   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

   The instance's values will be different for separate threads.

   For more details and extensive examples, see the documentation
   string of the ``_threading_local`` module.

threading.Lock()

   A factory function that returns a new primitive lock object.  Once
   a thread has acquired it, subsequent attempts to acquire it block,
   until it is released; any thread may release it.

   See *Lock Objects*.

threading.RLock()

   A factory function that returns a new reentrant lock object. 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.

   See *RLock Objects*.

threading.Semaphore(value=1)

   A factory function that returns a new semaphore object.  A
   semaphore manages a 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.

   See *Semaphore Objects*.

threading.BoundedSemaphore(value=1)

   A factory function that returns a new bounded semaphore object.  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.

class class threading.Thread

   A class that represents a thread of control.  This class can be
   safely subclassed in a limited fashion.

   See *Thread Objects*.

class class threading.Timer

   A thread that executes a function after a specified interval has
   passed.

   See *Timer Objects*.

threading.settrace(func)

   Set a trace function for all threads started from the ``threading``
   module. The *func* will be passed to  ``sys.settrace()`` for each
   thread, before its ``run()`` method is called.

threading.setprofile(func)

   Set a profile function for all threads started from the
   ``threading`` module. The *func* will be passed to
   ``sys.setprofile()`` for each thread, before its ``run()`` method
   is called.

threading.stack_size([size])

   Return the thread stack size used when creating new threads.  The
   optional *size* argument specifies the stack size to be used for
   subsequently created threads, and must be 0 (use platform or
   configured default) or a positive integer value of at least 32,768
   (32kB). If changing the thread stack size is unsupported, a
   ``ThreadError`` is raised.  If the specified stack size is invalid,
   a ``ValueError`` is raised and the stack size is unmodified.  32kB
   is currently the minimum supported stack size value to guarantee
   sufficient stack space for the interpreter itself.  Note that some
   platforms may have particular restrictions on values for the stack
   size, such as requiring a minimum stack size > 32kB or requiring
   allocation in multiples of the system memory page size - platform
   documentation should be referred to for more information (4kB pages
   are common; using multiples of 4096 for the stack size is the
   suggested approach in the absence of more specific information).
   Availability: Windows, systems with POSIX threads.

This module also defines the following constant:

threading.TIMEOUT_MAX

   The maximum value allowed for the *timeout* parameter of blocking
   functions (``Lock.acquire()``, ``RLock.acquire()``,
   ``Condition.wait()``, etc.). Specifying a timeout greater than this
   value will raise an ``OverflowError``.

   New in version 3.2.

Detailed interfaces for the objects are documented below.

The design of this module is loosely based on Java's threading model.
However, where Java makes locks and condition variables basic behavior
of every object, they are separate objects in Python.  Python's
``Thread`` class supports a subset of the behavior of Java's Thread
class; currently, there are no priorities, no thread groups, and
threads cannot be destroyed, stopped, suspended, resumed, or
interrupted.  The static methods of Java's Thread class, when
implemented, are mapped to module-level functions.

All of the methods described below are executed atomically.


Thread Objects
==============

This 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.

Other threads can call a thread's ``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`` attribute.

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.

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

   This constructor should always be called with keyword arguments.
   Arguments are:

   *group* should be ``None``; reserved for future extension when a
   ``ThreadGroup`` class is implemented.

   *target* is the callable object to be invoked by the ``run()``
   method. Defaults to ``None``, meaning nothing is called.

   *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 the subclass overrides the constructor, it must make sure to
   invoke the base class constructor (``Thread.__init__()``) before
   doing anything else to the thread.

   start()

      Start the thread's activity.

      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.

      This method will raise a ``RuntimeError`` if called more than
      once on the same thread object.

   run()

      Method representing the thread's activity.

      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.

      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). 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.

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

      A thread can be ``join()``ed many times.

      ``join()`` raises a ``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 raises 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()

      Old getter/setter API for ``name``; use it directly as a
      property instead.

   ident

      The 'thread identifier' of this thread or ``None`` if the thread
      has not been started.  This is a nonzero integer.  See the
      ``thread.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()

      Return whether the thread is alive.

      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()

      Old getter/setter API for ``daemon``; use it directly as a
      property instead.


Lock Objects
============

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`` will be raised.

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.

Lock.acquire(blocking=True, timeout=-1)

   Acquire a lock, blocking or non-blocking.

   When invoked without arguments, block until the lock is unlocked,
   then set it to locked, and return true.

   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.  A
   negative *timeout* argument specifies an unbounded wait.  It is
   forbidden to specify a *timeout* when *blocking* is false.

   The return value is ``True`` if the lock is acquired successfully,
   ``False`` if not (for example if the *timeout* expired).

   Changed in version 3.2: The *timeout* parameter is new.

   Changed in version 3.2: Lock acquires can now be interrupted by
   signals on POSIX.

Lock.release()

   Release a 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.

   Do not call this method when the lock is unlocked.

   There is no return value.


RLock Objects
=============

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()`` method. ``acquire()``/``release()`` call pairs
may be nested; only the final ``release()`` (the ``release()`` of the
outermost pair) resets the lock to unlocked and allows another thread
blocked in ``acquire()`` to proceed.

RLock.acquire(blocking=True, timeout=-1)

   Acquire a lock, blocking or non-blocking.

   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.

   Changed in version 3.2: The *timeout* parameter is new.

RLock.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.

   There is no return value.


Condition Objects
=================

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.)

A condition variable has ``acquire()`` and ``release()`` methods that
call the corresponding methods of the associated lock. It also has a
``wait()`` method, and ``notify()`` and ``notify_all()`` methods.
These three must only be called when the calling thread has acquired
the lock, otherwise a ``RuntimeError`` is raised.

The ``wait()`` method releases the lock, and then blocks until it is
awakened by a ``notify()`` or ``notify_all()`` call for the same
condition variable in another thread.  Once awakened, it re-acquires
the lock and returns.  It is also possible to specify a timeout.

The ``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.

Tip: 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
   cv.acquire()
   while not an_item_is_available():
       cv.wait()
   get_an_available_item()
   cv.release()

   # Produce one item
   cv.acquire()
   make_an_item_available()
   cv.notify()
   cv.release()

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.

Note:  Condition variables can be, depending on the implementation,
subject to both spurious wakeups (when ``wait()`` returns without a
``notify()`` call) and stolen wakeups (when another thread acquires
the lock before the awoken thread.)  For this reason, it is always
necessary to verify the state the thread is waiting for when
``wait()`` returns and optionally repeat the call as often as
necessary.

class class threading.Condition(lock=None)

   If the *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.

   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`` is raised.

      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.

      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).

      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.

      The return value is ``True`` unless a given *timeout* expired,
      in which case it is ``False``.

      Changed in version 3.2: Previously, the method always returned
      ``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-aquired on return.  The
      predicate is evaluated with the lock held.

      Using this method, the consumer example above can be written
      thus:

         with cv:
             cv.wait_for(an_item_is_available)
             get_an_available_item()

      New in version 3.2.

   notify()

      Wake up a thread waiting on this condition, if any.  If the
      calling thread has not acquired the lock when this method is
      called, a ``RuntimeError`` is raised.

      This method wakes up one of the threads waiting for the
      condition variable, if any are waiting; it is a no-op if no
      threads are waiting.

      The current implementation wakes up exactly one thread, if any
      are waiting.  However, it's not safe to rely on this behavior.
      A future, optimized implementation may occasionally wake up more
      than one thread.

      Note: the 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`` is raised.


Semaphore Objects
=================

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 ``P()`` and ``V()`` instead of
``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()``.

class class threading.Semaphore(value=1)

   The optional argument gives the initial *value* for the internal
   counter; it defaults to ``1``. If the *value* given is less than 0,
   ``ValueError`` is raised.

   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
      immediately.  If it is zero on entry, block, waiting until some
      other thread has called ``release()`` to make it larger than
      zero.  This is done with proper interlocking so that if multiple
      ``acquire()`` calls are blocked, ``release()`` will wake exactly
      one of them up.  The implementation may pick one at random, so
      the order in which blocked threads are awakened should not be
      relied on.  Returns true (or blocks indefinitely).

      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.

      Changed in version 3.2: The *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.


``Semaphore`` Example
---------------------

Semaphores are often used to guard resources with limited capacity,
for example, a database server.  In any situation where the size of
the resource is fixed, you should use a bounded semaphore.  Before
spawning any worker threads, your main thread would initialize the
semaphore:

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

Once spawned, worker threads call the semaphore's acquire and release
methods when they need to connect to the server:

   pool_sema.acquire()
   conn = connectdb()
   ... use connection ...
   conn.close()
   pool_sema.release()

The use of a bounded semaphore reduces the chance that a programming
error which causes the semaphore to be released more than it's
acquired will go undetected.


Event Objects
=============

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 class threading.Event

   The internal flag is initially false.

   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 the internal flag on exit, so it will always
      return ``True`` except if a timeout is given and the operation
      times out.

      Changed in version 3.1: Previously, the method always returned
      ``None``.


Timer Objects
=============

This class represents an action that should be run only after a
certain amount of time has passed --- a timer.  ``Timer`` is a
subclass of ``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.

For example:

   def hello():
       print("hello, world")

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

class class threading.Timer(interval, function, args=[], kwargs={})

   Create a timer that will run *function* with arguments *args* and
   keyword arguments *kwargs*, after *interval* seconds have passed.

   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.


Barrier Objects
===============

New in version 3.2.

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 the call.  At this
points, the threads are released simultanously.

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 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()`` method.

   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, is 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`` exception.

      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 exception threading.BrokenBarrierError

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


Using locks, conditions, and semaphores in the ``with`` statement
=================================================================

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.

Currently, ``Lock``, ``RLock``, ``Condition``, ``Semaphore``, and
``BoundedSemaphore`` objects may be used as ``with`` statement context
managers.  For example:

   import threading

   some_rlock = threading.RLock()

   with some_rlock:
       print("some_rlock is locked while this executes")


Importing in threaded code
==========================

While the import machinery is thread-safe, there are two key
restrictions on threaded imports due to inherent limitations in the
way that thread-safety is provided:

* Firstly, other than in the main module, an import should not have
  the side effect of spawning a new thread and then waiting for that
  thread in any way. Failing to abide by this restriction can lead to
  a deadlock if the spawned thread directly or indirectly attempts to
  import a module.

* Secondly, all import attempts must be completed before the
  interpreter starts shutting itself down. This can be most easily
  achieved by only performing imports from non-daemon threads created
  through the threading module. Daemon threads and threads created
  directly with the thread module will require some other form of
  synchronization to ensure they do not attempt imports after system
  shutdown has commenced. Failure to abide by this restriction will
  lead to intermittent exceptions and crashes during interpreter
  shutdown (as the late imports attempt to access machinery which is
  no longer in a valid state).
