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:mod:`threading` --- Thread-based parallelism

.. module:: threading
   :synopsis: Thread-based parallelism.

Source code: :source:`Lib/threading.py`


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

.. versionchanged:: 3.7
   This module used to be optional, it is now always available.

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.

This module defines the following functions:

.. function:: active_count()

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


.. function:: current_thread()

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


.. function:: get_ident()

   Return the 'thread identifier' of the current thread.  This is a nonzero
   integer.  Its value has no direct meaning; it is intended as a magic cookie
   to be used e.g. to index a dictionary of thread-specific data.  Thread
   identifiers may be recycled when a thread exits and another thread is
   created.

   .. versionadded:: 3.3


.. function:: enumerate()

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


.. function:: main_thread()

   Return the main :class:`Thread` object.  In normal conditions, the
   main thread is the thread from which the Python interpreter was
   started.

   .. versionadded:: 3.4


.. function:: settrace(func)

   .. index:: single: trace function

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


.. function:: setprofile(func)

   .. index:: single: profile function

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


.. function:: 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 (32 KiB). If *size* is not specified,
   0 is used.  If changing the thread stack size is
   unsupported, a :exc:`RuntimeError` is raised.  If the specified stack size is
   invalid, a :exc:`ValueError` is raised and the stack size is unmodified.  32 KiB
   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 > 32 KiB or requiring allocation in multiples of the system
   memory page size - platform documentation should be referred to for more
   information (4 KiB 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:

.. data:: TIMEOUT_MAX

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

   .. versionadded:: 3.2


This module defines a number of classes, which are detailed in the sections 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 :class:`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-Local Data

Thread-local data is data whose values are thread specific. To manage thread-local data, just create an instance of :class:`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.

A class that represents thread-local data.

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

Thread Objects

The :class:`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 :meth:`~Thread.run` method in a subclass. No other methods (except for the constructor) should be overridden in a subclass. In other words, only override the :meth:`~Thread.__init__` and :meth:`~Thread.run` methods of this class.

Once a thread object is created, its activity must be started by calling the thread's :meth:`~Thread.start` method. This invokes the :meth:`~Thread.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 :meth:`~Thread.run` method terminates -- either normally, or by raising an unhandled exception. The :meth:`~Thread.is_alive` method tests whether the thread is alive.

Other threads can call a thread's :meth:`~Thread.join` method. This blocks the calling thread until the thread whose :meth:`~Thread.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 :attr:`~Thread.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 :attr:`~Thread.daemon` property or the daemon constructor argument.

Note

Daemon threads are abruptly stopped at shutdown. Their resources (such as open files, database transactions, etc.) may not be released properly. If you want your threads to stop gracefully, make them non-daemonic and use a suitable signalling mechanism such as an :class:`Event`.

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 :meth:`~Thread.join`ed. They are never deleted, since it is impossible to detect the termination of alien threads.

.. impl-detail::

   In CPython, due to the :term:`Global Interpreter Lock`, 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 use of the computational
   resources of multi-core machines, you are advised to use
   :mod:`multiprocessing` or :class:`concurrent.futures.ProcessPoolExecutor`.
   However, threading is still an appropriate model if you want to run
   multiple I/O-bound tasks simultaneously.


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 :mod:`_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, :meth:`~Lock.acquire` and :meth:`~Lock.release`. When the state is unlocked, :meth:`~Lock.acquire` changes the state to locked and returns immediately. When the state is locked, :meth:`~Lock.acquire` blocks until a call to :meth:`~Lock.release` in another thread changes it to unlocked, then the :meth:`~Lock.acquire` call resets it to locked and returns. The :meth:`~Lock.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 :exc:`RuntimeError` will be raised.

Locks also support the :ref:`context management protocol <with-locks>`.

When more than one thread is blocked in :meth:`~Lock.acquire` waiting for the state to turn to unlocked, only one thread proceeds when a :meth:`~Lock.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.

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.

Note that 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.

.. method:: acquire(blocking=True, timeout=-1)

   Acquire a lock, blocking or non-blocking.

   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* set to ``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* is false.

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

   .. versionchanged:: 3.2
      The *timeout* parameter is new.

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


.. method:: 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 :exc:`RuntimeError` is raised.

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

Reentrant locks also support the :ref:`context management protocol <with-locks>`.

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.

Note that 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.

.. method:: 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.

   .. versionchanged:: 3.2
      The *timeout* parameter is new.


.. method:: 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
   :exc:`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. The lock is part of the condition object: you don't have to track it separately.

A condition variable obeys the :ref:`context management protocol <with-locks>`: using the with statement acquires the associated lock for the duration of the enclosed block. The :meth:`~Condition.acquire` and :meth:`~Condition.release` methods also call the corresponding methods of the associated lock.

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

The :meth:`~Condition.notify` method wakes up one of the threads waiting for the condition variable, if any are waiting. The :meth:`~Condition.notify_all` method wakes up all threads waiting for the condition variable.

Note: the :meth:`~Condition.notify` and :meth:`~Condition.notify_all` methods don't release the lock; this means that the thread or threads awakened will not return from their :meth:`~Condition.wait` call immediately, but only when the thread that called :meth:`~Condition.notify` or :meth:`~Condition.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 :meth:`~Condition.wait` repeatedly until they see the desired state, while threads that modify the state call :meth:`~Condition.notify` or :meth:`~Condition.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()

The while loop checking for the application's condition is necessary because :meth:`~Condition.wait` can return after an arbitrary long time, and the condition which prompted the :meth:`~Condition.notify` call may no longer hold true. This is inherent to multi-threaded programming. The :meth:`~Condition.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 :meth:`~Condition.notify` and :meth:`~Condition.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.

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

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

.. versionchanged:: 3.3
   changed from a factory function to a class.

.. method:: acquire(*args)

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

.. method:: release()

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

.. method:: 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 :exc:`RuntimeError` is
   raised.

   This method releases the underlying lock, and then blocks until it is
   awakened by a :meth:`notify` or :meth:`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 :class:`RLock`, it is not released using
   its :meth:`release` method, since this may not actually unlock the lock
   when it was acquired multiple times recursively.  Instead, an internal
   interface of the :class:`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``.

   .. versionchanged:: 3.2
      Previously, the method always returned ``None``.

.. method:: 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 :meth:`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 :meth:`wait`: The lock must be
   held when called and is re-acquired on return.  The predicate is evaluated
   with the lock held.

   .. versionadded:: 3.2

.. method:: 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
   :exc:`RuntimeError` is raised.

   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 :meth:`wait`
   call until it can reacquire the lock.  Since :meth:`notify` does not
   release the lock, its caller should.

.. method:: notify_all()

   Wake up all threads waiting on this condition.  This method acts like
   :meth:`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
   :exc:`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 the names P() and V() instead of :meth:`~Semaphore.acquire` and :meth:`~Semaphore.release`).

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

Semaphores also support the :ref:`context management protocol <with-locks>`.

This class implements semaphore objects. A semaphore manages an atomic counter representing the number of :meth:`release` calls minus the number of :meth:`acquire` calls, plus an initial value. The :meth:`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. If the value given is less than 0, :exc:`ValueError` is raised.

.. versionchanged:: 3.3
   changed from a factory function to a class.

.. method:: 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
     :meth:`~Semaphore.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 :meth:`~Semaphore.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.

   .. versionchanged:: 3.2
      The *timeout* parameter is new.

.. method:: 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 implementing bounded semaphore objects. A bounded semaphore checks to make sure its current value doesn't exceed its initial value. If it does, :exc:`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.

.. versionchanged:: 3.3
   changed from a factory function to a class.

:class:`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:

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

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 :meth:`~Event.set` method and reset to false with the :meth:`~Event.clear` method. The :meth:`~Event.wait` method blocks until the flag is true.

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

.. versionchanged:: 3.3
   changed from a factory function to a class.

.. method:: is_set()

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

.. method:: set()

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

.. method:: clear()

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

.. method:: 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
   :meth:`.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.

   .. versionchanged:: 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. :class:`Timer` is a subclass of :class:`Thread` and as such also functions as an example of creating custom threads.

Timers are started, as with threads, by calling their :meth:`~Timer.start` method. The timer can be stopped (before its action has begun) by calling the :meth:`~Timer.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

Create a timer that will run function with arguments args and keyword arguments 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.

.. versionchanged:: 3.3
   changed from a factory function to a class.

.. method:: 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

.. versionadded:: 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 :meth:`~Barrier.wait` method and will block until all of the threads have made their :meth:`~Barrier.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)

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 :meth:`wait` method.

.. 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, 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 :class:`BrokenBarrierError` exception if the
   barrier is broken or reset while a thread is waiting.

.. method:: reset()

   Return the barrier to the default, empty state.  Any threads waiting on it
   will receive the :class:`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.

.. method:: abort()

   Put the barrier into a broken state.  This causes any active or future
   calls to :meth:`wait` to fail with the :class:`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.

.. attribute:: parties

   The number of threads required to pass the barrier.

.. attribute:: n_waiting

   The number of threads currently waiting in the barrier.

.. attribute:: broken

   A boolean that is ``True`` if the barrier is in the broken state.
.. exception:: BrokenBarrierError

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


Using locks, conditions, and semaphores in the :keyword:`with` statement

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

with some_lock:
    # do something...

is equivalent to:

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

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