features/pep-420 / Doc / library / threading.rst

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

Source code: :source:`Lib/`

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.


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 and objects:

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.

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

See :ref:`thread-objects`.

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

See :ref:`timer-objects`.

This module also defines the following constant:

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

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.

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.

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:`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 state to unlocked and returns immediately. If an attempt is made to release an unlocked lock, a :exc:`RuntimeError` will be raised.

When more than one thread is blocked in :meth:`acquire` waiting for the state to 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.

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

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:`notify_all` methods. These three must only 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:`notify_all` call for the same condition variable in another thread. Once awakened, it re-acquires the lock and returns. It is also possible to specify a timeout.

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

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

Tip: the typical programming style using condition variables uses the lock to synchronize access to some shared state; threads that are interested in a particular change of state call :meth:`wait` repeatedly until they see the desired state, while threads that modify the state call :meth:`notify` or :meth:`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
while not an_item_is_available():

# Produce one item

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

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

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.

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 :meth:`P` and :meth:`V` instead of :meth:`acquire` and :meth:`release`).

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

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.

: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:

conn = connectdb()
... use connection ...

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:`clear` method. The :meth:`wait` method blocks until the flag is true.

The internal flag is initially false.

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

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

Barrier Objects

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:`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():
    while True:
        connection = accept_connection()

def client():
    while True:
        connection = make_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.

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.

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

import threading

some_rlock = threading.RLock()

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

Importing in threaded code

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

  • Firstly, other than in the main module, an import should not have the side effect of spawning a new thread and then waiting for that thread in any way. Failing to abide by this restriction can lead to a deadlock if the spawned thread directly or indirectly attempts to import a module.
  • Secondly, all import attempts must be completed before the interpreter starts shutting itself down. This can be most easily achieved by only performing imports from non-daemon threads created through the threading module. Daemon threads and threads created directly with the thread module will require some other form of synchronization to ensure they do not attempt imports after system shutdown has commenced. Failure to abide by this restriction will lead to intermittent exceptions and crashes during interpreter shutdown (as the late imports attempt to access machinery which is no longer in a valid state).