n | |
| :mod:`threading` --- Higher-level threading interface |
| ===================================================== |
| |
| .. module:: threading |
| :synopsis: Higher-level threading interface. |
| |
| |
| This module constructs higher-level threading interfaces on top of the lower |
| level :mod:`thread` module. |
n | See also the :mod:`mutex` and :mod:`Queue` modules. |
| |
| The :mod:`dummy_threading` module is provided for situations where |
| :mod:`threading` cannot be used because :mod:`thread` is missing. |
| |
n | .. note:: |
| |
| Starting with Python 2.6, this module provides PEP 8 compliant aliases and |
| properties to replace the ``camelCase`` names that were inspired by Java's |
| threading API. This updated API is compatible with that of the |
| :mod:`multiprocessing` module. However, no schedule has been set for the |
| deprecation of the ``camelCase`` names and they remain fully supported in |
| both Python 2.x and 3.x. |
| |
| .. note:: |
| |
| Starting with Python 2.5, several Thread methods raise :exc:`RuntimeError` |
| instead of :exc:`AssertionError` if called erroneously. |
| |
| |
| This module defines the following functions and objects: |
| |
n | |
| .. function:: activeCount() |
| .. function:: active_count() |
| activeCount() |
| |
n | Return the number of currently active :class:`Thread` objects. The returned |
n | Return the number of :class:`Thread` objects currently alive. The returned |
| count is equal to the length of the list returned by :func:`enumerate`. A |
| count is equal to the length of the list returned by :func:`enumerate`. |
| function that returns the number of currently active threads. |
| |
| |
| .. function:: Condition() |
n | :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. |
| |
| |
n | .. function:: currentThread() |
n | .. function:: current_thread() |
| currentThread() |
| |
| 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() |
| |
n | Return a list of all currently active :class:`Thread` objects. The list includes |
n | Return a list of all :class:`Thread` objects currently alive. The list |
| daemonic threads, dummy thread objects created by :func:`currentThread`, and the |
| includes daemonic threads, dummy thread objects created by |
| main thread. It excludes terminated threads and threads that have not yet been |
| :func:`current_thread`, and the main thread. It excludes terminated threads |
| started. |
| and threads that have not yet been started. |
| |
| |
| .. function:: Event() |
n | :noindex: |
| |
| A factory function that returns a new event object. An event manages a flag |
| that can be set to true with the :meth:`set` method and reset to false with the |
| :meth:`clear` method. The :meth:`wait` method blocks until the flag is true. |
| |
| |
| .. class:: local |
| |
| 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. |
| |
| |
n | .. _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:`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:`__init__` and :meth:`run` methods of |
| this class. |
| |
| Once a thread object is created, its activity must be started by calling the |
| thread's :meth:`start` method. This invokes the :meth:`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:`run` method terminates -- either normally, or |
| by raising an unhandled exception. The :meth:`is_alive` method tests whether the |
| thread is alive. |
| |
| Other threads can call a thread's :meth:`join` method. This blocks the calling |
| thread until the thread whose :meth:`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:`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:`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:`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:: Thread.start() |
| |
| Start the thread's activity. |
| |
| It must be called at most once per thread object. It arranges for the object's |
| :meth:`run` method to be invoked in a separate thread of control. |
| |
| This method will raise a :exc:`RuntimeException` if called more than once on the |
| same thread object. |
| |
| |
| .. method:: Thread.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:: Thread.join([timeout]) |
| |
| Wait until the thread terminates. This blocks the calling thread until the |
| thread whose :meth:`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:`join` always returns ``None``, you must call :meth:`isAlive` |
| after :meth:`join` to decide whether a timeout happened -- if the thread is |
| still alive, the :meth:`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:`join`\ ed many times. |
| |
| :meth:`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:`join` a thread before it has been started and attempts to do so |
| raises the same exception. |
| |
| |
| .. method:: Thread.getName() |
| Thread.setName() |
| |
| Old API for :attr:`~Thread.name`. |
| |
| |
| .. attribute:: Thread.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. |
| |
| |
| .. attribute:: Thread.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. |
| |
| .. versionadded:: 2.6 |
| |
| |
| .. method:: Thread.is_alive() |
| Thread.isAlive() |
| |
| Return whether the thread is alive. |
| |
| Roughly, a thread is alive from the moment the :meth:`start` method returns |
| until its :meth:`run` method terminates. The module function :func:`enumerate` |
| returns a list of all alive threads. |
| |
| |
| .. method:: Thread.isDaemon() |
| Thread.setDaemon() |
| |
| Old API for :attr:`~Thread.daemon`. |
| |
| |
| .. attribute:: Thread.daemon |
| |
| A boolean value indicating whether this thread is a daemon thread (True) or |
| not (False). This must be set before :meth:`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:`daemon` = ``False``. |
| |
| The entire Python program exits when no alive non-daemon threads are left. |
| |
| |
| .. _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` |
| |
| 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:`acquire` and |
| :meth:`release`. When the state is unlocked, :meth:`acquire` changes the state |
| to locked and returns immediately. When the state is locked, :meth:`acquire` |
| blocks until a call to :meth:`release` in another thread changes it to unlocked, |
| then the :meth:`acquire` call resets it to locked and returns. The |
| :meth:`release` method should only be called in the locked state; it changes the |
n | state to unlocked and returns immediately. When more than one thread is blocked |
n | state to unlocked and returns immediately. If an attempt is made to release an |
| in :meth:`acquire` waiting for the state to turn to unlocked, only one thread |
| unlocked lock, a :exc:`RuntimeError` will be raised. |
| proceeds when a :meth:`release` call resets the state to unlocked; which one of |
| |
| the waiting threads proceeds is not defined, and may vary across |
| When more than one thread is blocked in :meth:`acquire` waiting for the state to |
| implementations. |
| turn to unlocked, only one thread proceeds when a :meth:`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. |
| |
| |
n | .. method:: XXX Class.acquire([blocking\ ``= 1``]) |
n | .. method:: Lock.acquire([blocking=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. |
| |
| |
n | .. method:: XXX Class.release() |
n | .. method:: 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. |
| 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. |
| |
| |
n | .. method:: XXX Class.release() |
n | .. 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. |
| |
n | Only call this method when the calling thread owns the lock. Do not call this |
n | Only call this method when the calling thread owns the lock. A |
| method when the lock is unlocked. |
| :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.) |
| |
| A condition variable has :meth:`acquire` and :meth:`release` methods that call |
| the corresponding methods of the associated lock. It also has a :meth:`wait` |
| method, and :meth:`notify` and :meth:`notifyAll` methods. These three must only |
n | be called when the calling thread has acquired the lock. |
n | be called when the calling thread has acquired the lock, otherwise a |
| :exc:`RuntimeError` is raised. |
| |
| The :meth:`wait` method releases the lock, and then blocks until it is awakened |
| by a :meth:`notify` or :meth:`notifyAll` 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 :meth:`notify` method wakes up one of the threads waiting for the condition |
| variable, if any are waiting. The :meth:`notifyAll` method wakes up all threads |
| 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. |
| |
| |
| .. method:: Condition.notify() |
| |
n | Wake up a thread waiting on this condition, if any. This must only be called |
n | Wake up a thread waiting on this condition, if any. Wait until notified or until |
| when the calling thread has acquired the lock. |
| a timeout occurs. If the calling thread has not acquired the lock when this |
| method is called, a :exc:`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 :meth:`wait` call |
| until it can reacquire the lock. Since :meth:`notify` does not release the |
| lock, its caller should. |
| |
| |
n | .. method:: Condition.notifyAll() |
n | .. method:: Condition.notify_all() |
| Condition.notifyAll() |
| |
| Wake up all threads waiting on this condition. This method acts like |
n | :meth:`notify`, but wakes up all waiting threads instead of one. |
n | :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 |
| |
| 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). |
n | |
| |
| .. _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:`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:`__init__` and :meth:`run` methods of |
| this class. |
| |
| Once a thread object is created, its activity must be started by calling the |
| thread's :meth:`start` method. This invokes the :meth:`run` method in a |
| separate thread of control. |
| |
| Once the thread's activity is started, the thread is considered 'alive' and |
| 'active' (these concepts are almost, but not quite exactly, the same; their |
| definition is intentionally somewhat vague). It stops being alive and active |
| when its :meth:`run` method terminates -- either normally, or by raising an |
| unhandled exception. The :meth:`isAlive` method tests whether the thread is |
| alive. |
| |
| Other threads can call a thread's :meth:`join` method. This blocks the calling |
| thread until the thread whose :meth:`join` method is called is terminated. |
| |
| A thread has a name. The name can be passed to the constructor, set with the |
| :meth:`setName` method, and retrieved with the :meth:`getName` method. |
| |
| 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 with |
| the :meth:`setDaemon` method and retrieved with the :meth:`isDaemon` method. |
| |
| 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". These 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, |
| active, and daemonic, and cannot be :meth:`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:: Thread.start() |
| |
| Start the thread's activity. |
| |
| This must be called at most once per thread object. It arranges for the |
| object's :meth:`run` method to be invoked in a separate thread of control. |
| |
| |
| .. method:: Thread.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:: Thread.join([timeout]) |
| |
| Wait until the thread terminates. This blocks the calling thread until the |
| thread whose :meth:`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:`join` always returns ``None``, you must call |
| :meth:`isAlive` to decide whether a timeout happened. |
| |
| When the *timeout* argument is not present or ``None``, the operation will block |
| until the thread terminates. |
| |
| A thread can be :meth:`join`\ ed many times. |
| |
| A thread cannot join itself because this would cause a deadlock. |
| |
| It is an error to attempt to :meth:`join` a thread before it has been started. |
| |
| |
| .. method:: Thread.getName() |
| |
| Return the thread's name. |
| |
| |
| .. method:: Thread.setName(name) |
| |
| Set the thread's name. |
| |
| The name is 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:: Thread.isAlive() |
| |
| Return whether the thread is alive. |
| |
| Roughly, a thread is alive from the moment the :meth:`start` method returns |
| until its :meth:`run` method terminates. |
| |
| |
| .. method:: Thread.isDaemon() |
| |
| Return the thread's daemon flag. |
| |
| |
| .. method:: Thread.setDaemon(daemonic) |
| |
| Set the thread's daemon flag to the Boolean value *daemonic*. This must be |
| called before :meth:`start` is called. |
| |
| The initial value is inherited from the creating thread. |
| |
| The entire Python program exits when no active non-daemon threads are left. |
| |
| |
| .. _timer-objects: |
| |
| Timer Objects |
| ------------- |
| |
| This class represents an action that should be run only after a certain amount |
| :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. |
| |
| Currently, :class:`Lock`, :class:`RLock`, :class:`Condition`, |
| :class:`Semaphore`, and :class:`BoundedSemaphore` objects may be used as |
| :keyword:`with` statement context managers. For example:: |
| |
n | from __future__ import with_statement |
| import threading |
| |
| some_rlock = threading.RLock() |
| |
| with some_rlock: |
| print "some_rlock is locked while this executes" |
| |
t | |
| .. _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). |