| :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. |
| |
| The :mod:`dummy_threading` module is provided for situations where |
| :mod:`threading` cannot be used because :mod:`_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. |
| |
| .. 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. |
| |
| |
| This module defines the following functions and objects: |
| |
| |
| .. 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:: Condition() |
| :noindex: |
| |
| 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 :ref:`condition-objects`. |
| |
| |
| .. 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:: 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:: Event() |
| :noindex: |
| |
| A factory function that returns a new event object. 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. |
| |
| See :ref:`event-objects`. |
| |
| |
| .. class:: 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 :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. |
| |
| For more details and extensive examples, see the documentation string of the |
| :mod:`_threading_local` module. |
| |
| |
| .. function:: 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 :ref:`lock-objects`. |
| |
| |
| .. function:: 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 :ref:`rlock-objects`. |
| |
| |
| .. function:: Semaphore(value=1) |
| :noindex: |
| |
| A factory function that returns a new semaphore object. A semaphore manages a |
| 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. |
| |
| See :ref:`semaphore-objects`. |
| |
| |
| .. function:: 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, :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. |
| |
| |
| .. class:: Thread |
| :noindex: |
| |
| A class that represents a thread of control. This class can be safely |
| subclassed in a limited fashion. |
| |
| See :ref:`thread-objects`. |
| |
| |
| .. class:: Timer |
| :noindex: |
| |
| A thread that executes a function after a specified interval has passed. |
| |
| See :ref:`timer-objects`. |
| |
| |
| .. 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:`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:`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 (32kB). If changing the thread stack size is |
| unsupported, a :exc:`ThreadError` is raised. If the specified stack size is |
| invalid, a :exc:`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: |
| |
| .. 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 |
| |
| |
| 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 :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-objects: |
| |
| 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 :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. |
| |
| 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. |
| |
| |
| .. class:: 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 |
| :class:`ThreadGroup` class is implemented. |
| |
| *target* is the callable object to be invoked by the :meth:`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. |
| |
| .. method:: start() |
| |
| Start the thread's activity. |
| |
| It must be called at most once per thread object. It arranges for the |
| object's :meth:`~Thread.run` method to be invoked in a separate thread |
| of control. |
| |
| This method will raise a :exc:`RuntimeError` if called more than once |
| on the same thread object. |
| |
| .. method:: run() |
| |
| Method representing the thread's activity. |
| |
| You may override this method in a subclass. The standard :meth:`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. |
| |
| .. method:: join(timeout=None) |
| |
| Wait until the thread terminates. This blocks the calling thread until |
| the thread whose :meth:`~Thread.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 :meth:`~Thread.join` always returns ``None``, |
| you must call :meth:`~Thread.is_alive` after :meth:`~Thread.join` to |
| decide whether a timeout happened -- if the thread is still alive, the |
| :meth:`~Thread.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 :meth:`~Thread.join`\ ed many times. |
| |
| :meth:`~Thread.join` raises a :exc:`RuntimeError` if an attempt is made |
| to join the current thread as that would cause a deadlock. It is also |
| an error to :meth:`~Thread.join` a thread before it has been started |
| and attempts to do so raise the same exception. |
| |
| .. attribute:: 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. |
| |
| .. method:: getName() |
| setName() |
| |
| Old getter/setter API for :attr:`~Thread.name`; use it directly as a |
| property instead. |
| |
| .. attribute:: ident |
| |
| The 'thread identifier' of this thread or ``None`` if the thread has not |
| been started. This is a nonzero integer. See the |
| :func:`_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. |
| |
| .. method:: is_alive() |
| |
| Return whether the thread is alive. |
| |
| This method returns ``True`` just before the :meth:`~Thread.run` method |
| starts until just after the :meth:`~Thread.run` method terminates. The |
| module function :func:`.enumerate` returns a list of all alive threads. |
| |
| .. attribute:: daemon |
| |
| A boolean value indicating whether this thread is a daemon thread (True) |
| or not (False). This must be set before :meth:`~Thread.start` is called, |
| otherwise :exc:`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 |
| :attr:`~Thread.daemon` = ``False``. |
| |
| The entire Python program exits when no alive non-daemon threads are left. |
| |
| .. method:: isDaemon() |
| setDaemon() |
| |
| Old getter/setter API for :attr:`~Thread.daemon`; use it directly as a |
| property instead. |
| |
| |
| .. _lock-objects: |
| |
| 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 manager 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. |
| |
| |
| .. method:: Lock.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 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). |
| |
| .. versionchanged:: 3.2 |
| The *timeout* parameter is new. |
| |
| .. versionchanged:: 3.2 |
| Lock acquires can now be interrupted by signals on POSIX. |
| |
| |
| .. method:: Lock.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:`ThreadError` is raised. |
| |
| There is no return value. |
| |
| |
| .. _rlock-objects: |
| |
| 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 manager protocol <with-locks>`. |
| |
| |
| .. method:: 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. |
| |
| .. versionchanged:: 3.2 |
| The *timeout* parameter is new. |
| |
| |
| .. method:: 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 |
| :exc:`RuntimeError` is raised if this method is called when the lock is |
| unlocked. |
| |
| There is no return value. |
| |
| |
| .. _condition-objects: |
| |
| 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 manager 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. |
| |
| |
| Usage |
| ^^^^^ |
| |
| 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. |
| |
| |
| Interface |
| ^^^^^^^^^ |
| |
| .. class:: Condition(lock=None) |
| |
| 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. |
| |
| .. 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-aquired 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: |
| |
| 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 manager protocol <with-locks>`. |
| |
| |
| .. class:: 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, :exc:`ValueError` is |
| raised. |
| |
| .. 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 immediately. If it is zero |
| on entry, block, waiting until some other thread has called |
| :meth:`~Semaphore.release` to make it larger than zero. This is done |
| with proper interlocking so that if multiple :meth:`acquire` calls are |
| blocked, :meth:`~Semaphore.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. |
| |
| .. 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. |
| |
| |
| .. _semaphore-examples: |
| |
| :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: |
| |
| 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:: Event() |
| |
| The internal flag is initially false. |
| |
| .. 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: |
| |
| 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:`start` method. The |
| timer can be stopped (before its action has begun) by calling the :meth:`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:: Timer(interval, function, args=[], kwargs={}) |
| |
| Create a timer that will run *function* with arguments *args* and keyword |
| arguments *kwargs*, after *interval* seconds have passed. |
| |
| .. 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 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:: 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 :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. |
| |
| |
| .. _with-locks: |
| |
| 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. |
| |
| |
| .. _threaded-imports: |
| |
| 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). |