threading
— Thread-based parallelism¶
Código-fonte: Lib/threading.py
This module constructs higher-level threading interfaces on top of the lower
level _thread
module.
Alterado na versão 3.7: Este módulo costumava ser opcional, agora está sempre disponível.
Ver também
concurrent.futures.ThreadPoolExecutor
offers a higher level interface
to push tasks to a background thread without blocking execution of the
calling thread, while still being able to retrieve their results when needed.
queue
provides a thread-safe interface for exchanging data between
running threads.
asyncio
offers an alternative approach to achieving task level
concurrency without requiring the use of multiple operating system threads.
Nota
In the Python 2.x series, this module contained camelCase
names
for some methods and functions. These are deprecated as of Python 3.10,
but they are still supported for compatibility with Python 2.5 and lower.
Detalhes da implementação do CPython: In CPython, due to the 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
multiprocessing
or concurrent.futures.ProcessPoolExecutor
.
However, threading is still an appropriate model if you want to run
multiple I/O-bound tasks simultaneously.
Availability: not Emscripten, not WASI.
Este módulo não funciona ou não está disponível em plataformas WebAssembly wasm32-emscripten
e wasm32-wasi
. Veja Plataformas WebAssembly para mais informações.
Este módulo define as seguintes funções:
- threading.active_count()¶
Retorna o número de objetos
Thread
atualmente ativos. A quantidade retornada é igual ao tamanho da lista retornada porenumerate()
.The function
activeCount
is a deprecated alias for this function.
- threading.current_thread()¶
Retorna o objeto
Thread
atual, correspondendo ao controle do chamador da thread. Se o controle do chamador da thread não foi criado através do módulothreading
, um objeto thread vazio, com funcionalidade limitada, é retornado.The function
currentThread
is a deprecated alias for this function.
- threading.excepthook(args, /)¶
Handle uncaught exception raised by
Thread.run()
.O argumento args tem os seguintes atributos:
exc_type: Tipo de exceção..
exc_value: Valor da exceção, pode ser
None
.exc_traceback: Pilha de execução da exceção, pode ser
None
.thread: Thread que levantou a exceção, pode ser
None
.
Se exc_type é
SystemExit
, a exceção é silenciosamente ignorada. Caso contrário, a exceção é exibida emsys.stderr
.Se esta função levantar uma exceção,
sys.excepthook()
é chamada para tratá-la.threading.excepthook()
pode ser substituída para controlar como exceções não capturadas levantadas porThread.run()
são tratadas.Storing exc_value using a custom hook can create a reference cycle. It should be cleared explicitly to break the reference cycle when the exception is no longer needed.
Storing thread using a custom hook can resurrect it if it is set to an object which is being finalized. Avoid storing thread after the custom hook completes to avoid resurrecting objects.
Ver também
sys.excepthook()
trata exceções não capturadas.Adicionado na versão 3.8.
- threading.__excepthook__¶
Holds the original value of
threading.excepthook()
. It is saved so that the original value can be restored in case they happen to get replaced with broken or alternative objects.Adicionado na versão 3.10.
- threading.get_ident()¶
Retorna o ‘identificador de thread’ do thread atual. Este é um número inteiro diferente de zero. Seu valor não tem significado direto; pretende-se que seja um cookie mágico para ser usado, por exemplo, para indexar um dicionário de dados específicos do thread. identificadores de thread podem ser reciclados quando um thread sai e outro é criado.
Adicionado na versão 3.3.
- threading.get_native_id()¶
Retorna a ID de thread integral nativa da thread atual atribuída pelo kernel. Este é um número inteiro não negativo. Seu valor pode ser usado para identificar exclusivamente essa thread específica em todo o sistema (até que a thread termine, após o que o valor poderá ser reciclado pelo sistema operacional).
Availability: Windows, FreeBSD, Linux, macOS, OpenBSD, NetBSD, AIX, DragonFlyBSD.
Adicionado na versão 3.8.
- threading.enumerate()¶
Return a list of all
Thread
objects currently active. The list includes daemonic threads and dummy thread objects created bycurrent_thread()
. It excludes terminated threads and threads that have not yet been started. However, the main thread is always part of the result, even when terminated.
- threading.main_thread()¶
Return the main
Thread
object. In normal conditions, the main thread is the thread from which the Python interpreter was started.Adicionado na versão 3.4.
- threading.settrace(func)¶
Set a trace function for all threads started from the
threading
module. The func will be passed tosys.settrace()
for each thread, before itsrun()
method is called.
- threading.settrace_all_threads(func)¶
Set a trace function for all threads started from the
threading
module and all Python threads that are currently executing.The func will be passed to
sys.settrace()
for each thread, before itsrun()
method is called.Adicionado na versão 3.12.
- threading.gettrace()¶
Obtém a função trace conforme definido por
settrace()
.Adicionado na versão 3.10.
- threading.setprofile(func)¶
Set a profile function for all threads started from the
threading
module. The func will be passed tosys.setprofile()
for each thread, before itsrun()
method is called.
- threading.setprofile_all_threads(func)¶
Set a profile function for all threads started from the
threading
module and all Python threads that are currently executing.The func will be passed to
sys.setprofile()
for each thread, before itsrun()
method is called.Adicionado na versão 3.12.
- threading.getprofile()¶
Obtém a função do criador de perfil conforme definido por
setprofile()
.Adicionado na versão 3.10.
- threading.stack_size([size])¶
Retorna o tamanho da pilha de threads usado ao criar novos threads. O argumento opcional size especifica o tamanho da pilha a ser usado para threads criados posteriormente e deve ser 0 (usar plataforma ou padrão configurado) ou um valor inteiro positivo de pelo menos 32.768 (32 KiB). Se size não for especificado, 0 será usado. Se a alteração do tamanho da pilha de threads não for suportada, uma
RuntimeError
será levantada. Se o tamanho da pilha especificado for inválido, umaValueError
será levantada e o tamanho da pilha não será modificado. Atualmente, 0 KiB é o valor mínimo de tamanho de pilha suportado para garantir espaço suficiente para o próprio interpretador. Observe que algumas plataformas podem ter restrições específicas sobre valores para o tamanho da pilha, como exigir um tamanho mínimo de pilha > 32 KiB ou exigir alocação em múltiplos do tamanho da página de memória do sistema – a documentação da plataforma deve ser consultada para obter mais informações (4 páginas KiB são comuns; usar múltiplos de 4096 para o tamanho da pilha é a abordagem sugerida na ausência de informações mais específicas).Availability: Windows, pthreads.
Plataformas Unix com suporte a threads POSIX.
This module also defines the following constant:
- threading.TIMEOUT_MAX¶
The maximum value allowed for the timeout parameter of blocking functions (
Lock.acquire()
,RLock.acquire()
,Condition.wait()
, etc.). Specifying a timeout greater than this value will raise anOverflowError
.Adicionado na versão 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 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 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.
- class threading.local¶
A class that represents thread-local data.
For more details and extensive examples, see the documentation string of the
_threading_local
module: Lib/_threading_local.py.
Thread Objects¶
The 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 run()
method in a subclass. No other methods (except for the constructor) should be
overridden in a subclass. In other words, only override the
__init__()
and run()
methods of this class.
Once a thread object is created, its activity must be started by calling the
thread’s start()
method. This invokes the run()
method in a separate thread of control.
Once the thread’s activity is started, the thread is considered ‘alive’. It
stops being alive when its run()
method terminates – either
normally, or by raising an unhandled exception. The is_alive()
method tests whether the thread is alive.
Other threads can call a thread’s join()
method. This blocks
the calling thread until the thread whose join()
method is
called is terminated.
A thread has a name. The name can be passed to the constructor, and read or
changed through the name
attribute.
If the run()
method raises an exception,
threading.excepthook()
is called to handle it. By default,
threading.excepthook()
ignores silently SystemExit
.
A thread can be flagged as a “daemon thread”. The significance of this flag is
that the entire Python program exits when only daemon threads are left. The
initial value is inherited from the creating thread. The flag can be set
through the daemon
property or the daemon constructor
argument.
Nota
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 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 joined. They are never deleted, since it is impossible to detect the termination of alien threads.
- class threading.Thread(group=None, target=None, name=None, args=(), kwargs={}, *, daemon=None)¶
This constructor should always be called with keyword arguments. Arguments are:
group should be
None
; reserved for future extension when aThreadGroup
class is implemented.target is the callable object to be invoked by the
run()
method. Defaults toNone
, 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, or “Thread-N (target)” where “target” is
target.__name__
if the target argument is specified.args is a list or tuple of arguments for the target invocation. Defaults to
()
.kwargs is a dictionary of keyword arguments for the target invocation. Defaults to
{}
.If not
None
, daemon explicitly sets whether the thread is daemonic. IfNone
(the default), the daemonic property is inherited from the current thread.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.Alterado na versão 3.3: Adicionado o parâmetro daemon.
Alterado na versão 3.10: Use the target name if name argument is omitted.
- start()¶
Start the thread’s activity.
It must be called at most once per thread object. It arranges for the object’s
run()
method to be invoked in a separate thread of control.This method will raise a
RuntimeError
if called more than once on the same thread object.
- run()¶
Method representing the thread’s activity.
You may override this method in a subclass. The standard
run()
method invokes the callable object passed to the object’s constructor as the target argument, if any, with positional and keyword arguments taken from the args and kwargs arguments, respectively.Using list or tuple as the args argument which passed to the
Thread
could achieve the same effect.Exemplo:
>>> from threading import Thread >>> t = Thread(target=print, args=[1]) >>> t.run() 1 >>> t = Thread(target=print, args=(1,)) >>> t.run() 1
- join(timeout=None)¶
Wait until the thread terminates. This blocks the calling thread until the thread whose
join()
method is called terminates – either normally or through an unhandled exception – or until the optional timeout occurs.When the timeout argument is present and not
None
, it should be a floating-point number specifying a timeout for the operation in seconds (or fractions thereof). Asjoin()
always returnsNone
, you must callis_alive()
afterjoin()
to decide whether a timeout happened – if the thread is still alive, thejoin()
call timed out.When the timeout argument is not present or
None
, the operation will block until the thread terminates.A thread can be joined many times.
join()
raises aRuntimeError
if an attempt is made to join the current thread as that would cause a deadlock. It is also an error tojoin()
a thread before it has been started and attempts to do so raise the same exception.
- name¶
A string used for identification purposes only. It has no semantics. Multiple threads may be given the same name. The initial name is set by the constructor.
- getName()¶
- setName()¶
Deprecated getter/setter API for
name
; use it directly as a property instead.Obsoleto desde a versão 3.10.
- ident¶
The ‘thread identifier’ of this thread or
None
if the thread has not been started. This is a nonzero integer. See theget_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.
- native_id¶
The Thread ID (
TID
) of this thread, as assigned by the OS (kernel). This is a non-negative integer, orNone
if the thread has not been started. See theget_native_id()
function. This value may be used to uniquely identify this particular thread system-wide (until the thread terminates, after which the value may be recycled by the OS).Nota
Similar to Process IDs, Thread IDs are only valid (guaranteed unique system-wide) from the time the thread is created until the thread has been terminated.
Availability: Windows, FreeBSD, Linux, macOS, OpenBSD, NetBSD, AIX, DragonFlyBSD.
Adicionado na versão 3.8.
- is_alive()¶
Return whether the thread is alive.
This method returns
True
just before therun()
method starts until just after therun()
method terminates. The module functionenumerate()
returns a list of all alive threads.
- daemon¶
A boolean value indicating whether this thread is a daemon thread (
True
) or not (False
). This must be set beforestart()
is called, otherwiseRuntimeError
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 todaemon
=False
.The entire Python program exits when no alive non-daemon threads are left.
Lock Objects¶
A primitive lock is a synchronization primitive that is not owned by a
particular thread when locked. In Python, it is currently the lowest level
synchronization primitive available, implemented directly by the _thread
extension module.
A primitive lock is in one of two states, “locked” or “unlocked”. It is created
in the unlocked state. It has two basic methods, acquire()
and
release()
. When the state is unlocked, acquire()
changes the state to locked and returns immediately. When the state is locked,
acquire()
blocks until a call to release()
in another
thread changes it to unlocked, then the acquire()
call resets it
to locked and returns. The release()
method should only be
called in the locked state; it changes the state to unlocked and returns
immediately. If an attempt is made to release an unlocked lock, a
RuntimeError
will be raised.
Locks also support the context management protocol.
When more than one thread is blocked in acquire()
waiting for the
state to turn to unlocked, only one thread proceeds when a release()
call resets the state to unlocked; which one of the waiting threads proceeds
is not defined, and may vary across implementations.
All methods are executed atomically.
- class threading.Lock¶
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.- acquire(blocking=True, timeout=-1)¶
Adquire uma trava, bloqueante ou não.
When invoked with the blocking argument set to
True
(the default), block until the lock is unlocked, then set it to locked and returnTrue
.When invoked with the blocking argument set to
False
, do not block. If a call with blocking set toTrue
would block, returnFalse
immediately; otherwise, set the lock to locked and returnTrue
.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 isFalse
.The return value is
True
if the lock is acquired successfully,False
if not (for example if the timeout expired).Alterado na versão 3.2: O parâmetro timeout é novo.
Alterado na versão 3.2: Lock acquisition can now be interrupted by signals on POSIX if the underlying threading implementation supports it.
- 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
RuntimeError
is raised.There is no return value.
- locked()¶
Return
True
if the lock is acquired.
Objetos RLock¶
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.
Threads call a lock’s acquire()
method to lock it,
and its release()
method to unlock it.
Nota
Reentrant locks support the context management protocol,
so it is recommended to use with
instead of manually calling
acquire()
and release()
to handle acquiring and releasing the lock for a block of code.
RLock’s acquire()
/release()
call pairs may be nested,
unlike Lock’s acquire()
/release()
. Only the final
release()
(the release()
of the outermost pair) resets
the lock to an unlocked state and allows another thread blocked in
acquire()
to proceed.
acquire()
/release()
must be used in pairs: each acquire
must have a release in the thread that has acquired the lock. Failing to
call release as many times the lock has been acquired can lead to deadlock.
- class threading.RLock¶
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.- acquire(blocking=True, timeout=-1)¶
Adquire uma trava, bloqueante ou não.
Ver também
- Using RLock as a context manager
Recommended over manual
acquire()
andrelease()
calls whenever practical.
When invoked with the blocking argument set to
True
(the default):If no thread owns the lock, acquire the lock and return immediately.
If another thread owns the lock, block until we are able to acquire lock, or timeout, if set to a positive float value.
If the same thread owns the lock, acquire the lock again, and return immediately. This is the difference between
Lock
andRLock
;Lock
handles this case the same as the previous, blocking until the lock can be acquired.
When invoked with the blocking argument set to
False
:If no thread owns the lock, acquire the lock and return immediately.
If another thread owns the lock, return immediately.
If the same thread owns the lock, acquire the lock again and return immediately.
In all cases, if the thread was able to acquire the lock, return
True
. If the thread was unable to acquire the lock (i.e. if not blocking or the timeout was reached) returnFalse
.If called multiple times, failing to call
release()
as many times may lead to deadlock. Consider usingRLock
as a context manager rather than calling acquire/release directly.Alterado na versão 3.2: O parâmetro timeout é novo.
- release()¶
Release a lock, decrementing the recursion level. If after the decrement it is zero, reset the lock to unlocked (not owned by any thread), and if any other threads are blocked waiting for the lock to become unlocked, allow exactly one of them to proceed. If after the decrement the recursion level is still nonzero, the lock remains locked and owned by the calling thread.
Only call this method when the calling thread owns the lock. A
RuntimeError
is raised if this method is called when the lock is not acquired.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 context management protocol:
using the with
statement acquires the associated lock for the duration of
the enclosed block. The acquire()
and
release()
methods also call the corresponding methods of
the associated lock.
Other methods must be called with the associated lock held. The
wait()
method releases the lock, and then blocks until
another thread awakens it by calling notify()
or
notify_all()
. Once awakened, wait()
re-acquires the lock and returns. It is also possible to specify a timeout.
The notify()
method wakes up one of the threads waiting for
the condition variable, if any are waiting. The notify_all()
method wakes up all threads waiting for the condition variable.
Note: the notify()
and notify_all()
methods
don’t release the lock; this means that the thread or threads awakened will
not return from their wait()
call immediately, but only when
the thread that called notify()
or notify_all()
finally relinquishes ownership of the lock.
The typical programming style using condition variables uses the lock to
synchronize access to some shared state; threads that are interested in a
particular change of state call wait()
repeatedly until they
see the desired state, while threads that modify the state call
notify()
or notify_all()
when they change
the state in such a way that it could possibly be a desired state for one
of the waiters. For example, the following code is a generic
producer-consumer situation with unlimited buffer capacity:
# Consume one item
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 wait()
can return after an arbitrary long time,
and the condition which prompted the notify()
call may
no longer hold true. This is inherent to multi-threaded programming. The
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 notify()
and notify_all()
,
consider whether one state change can be interesting for only one or several
waiting threads. E.g. in a typical producer-consumer situation, adding one
item to the buffer only needs to wake up one consumer thread.
- class threading.Condition(lock=None)¶
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 aLock
orRLock
object, and it is used as the underlying lock. Otherwise, a newRLock
object is created and used as the underlying lock.Alterado na versão 3.3: changed from a factory function to a class.
- acquire(*args)¶
Acquire the underlying lock. This method calls the corresponding method on the underlying lock; the return value is whatever that method returns.
- release()¶
Release the underlying lock. This method calls the corresponding method on the underlying lock; there is no return value.
- wait(timeout=None)¶
Wait until notified or until a timeout occurs. If the calling thread has not acquired the lock when this method is called, a
RuntimeError
is raised.This method releases the underlying lock, and then blocks until it is awakened by a
notify()
ornotify_all()
call for the same condition variable in another thread, or until the optional timeout occurs. Once awakened or timed out, it re-acquires the lock and returns.When the timeout argument is present and not
None
, it should be a floating-point number specifying a timeout for the operation in seconds (or fractions thereof).When the underlying lock is an
RLock
, it is not released using itsrelease()
method, since this may not actually unlock the lock when it was acquired multiple times recursively. Instead, an internal interface of theRLock
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 isFalse
.Alterado na versão 3.2: Previously, the method always returned
None
.
- wait_for(predicate, timeout=None)¶
Wait until a condition evaluates to true. predicate should be a callable which result will be interpreted as a boolean value. A timeout may be provided giving the maximum time to wait.
This utility method may call
wait()
repeatedly until the predicate is satisfied, or until a timeout occurs. The return value is the last return value of the predicate and will evaluate toFalse
if the method timed out.Ignoring the timeout feature, calling this method is roughly equivalent to writing:
while not predicate(): cv.wait()
Therefore, the same rules apply as with
wait()
: The lock must be held when called and is re-acquired on return. The predicate is evaluated with the lock held.Adicionado na versão 3.2.
- 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
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
wait()
call until it can reacquire the lock. Sincenotify()
does not release the lock, its caller should.
- notify_all()¶
Wake up all threads waiting on this condition. This method acts like
notify()
, but wakes up all waiting threads instead of one. If the calling thread has not acquired the lock when this method is called, aRuntimeError
is raised.The method
notifyAll
is a deprecated alias for this method.
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 acquire()
and
release()
).
A semaphore manages an internal counter which is decremented by each
acquire()
call and incremented by each release()
call. The counter can never go below zero; when acquire()
finds that it is zero, it blocks, waiting until some other thread calls
release()
.
Semaphores also support the context management protocol.
- class threading.Semaphore(value=1)¶
This class implements semaphore objects. A semaphore manages an atomic counter representing the number of
release()
calls minus the number ofacquire()
calls, plus an initial value. Theacquire()
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,ValueError
is raised.Alterado na versão 3.3: changed from a factory function to a class.
- 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
release()
. Once awoken (and the counter is greater than 0), decrement the counter by 1 and returnTrue
. Exactly one thread will be awoken by each call torelease()
. 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, returnFalse
immediately; otherwise, do the same thing as when called without arguments, and returnTrue
.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, returnFalse
. ReturnTrue
otherwise.Alterado na versão 3.2: O parâmetro timeout é novo.
- release(n=1)¶
Release a semaphore, incrementing the internal counter by n. When it was zero on entry and other threads are waiting for it to become larger than zero again, wake up n of those threads.
Alterado na versão 3.9: Added the n parameter to release multiple waiting threads at once.
- class threading.BoundedSemaphore(value=1)¶
Class implementing bounded semaphore objects. A bounded semaphore checks to make sure its current value doesn’t exceed its initial value. If it does,
ValueError
is raised. In most situations semaphores are used to guard resources with limited capacity. If the semaphore is released too many times it’s a sign of a bug. If not given, value defaults to 1.Alterado na versão 3.3: changed from a factory function to a class.
Exemplo Semaphore
¶
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
set()
method and reset to false with the clear()
method. The wait()
method blocks until the flag is true.
- class threading.Event¶
Class implementing event objects. An event manages a flag that can be set to true with the
set()
method and reset to false with theclear()
method. Thewait()
method blocks until the flag is true. The flag is initially false.Alterado na versão 3.3: changed from a factory function to a class.
- is_set()¶
Return
True
if and only if the internal flag is true.The method
isSet
is a deprecated alias for this method.
- set()¶
Set the internal flag to true. All threads waiting for it to become true are awakened. Threads that call
wait()
once the flag is true will not block at all.
- clear()¶
Reset the internal flag to false. Subsequently, threads calling
wait()
will block untilset()
is called to set the internal flag to true again.
- wait(timeout=None)¶
Block as long as the internal flag is false and the timeout, if given, has not expired. The return value represents the reason that this blocking method returned;
True
if returning because the internal flag is set to true, orFalse
if a timeout is given and the internal flag did not become true within the given wait time.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.Alterado na versão 3.1: Previously, the method always returned
None
.
Objetos Timer¶
This class represents an action that should be run only after a certain amount
of time has passed — a timer. Timer
is a subclass of Thread
and as such also functions as an example of creating custom threads.
Timers are started, as with threads, by calling their Timer.start
method. The timer can be stopped (before its action has begun) by calling the
cancel()
method. The interval the timer will wait before
executing its action may not be exactly the same as the interval specified by
the user.
Por exemplo:
def hello():
print("hello, world")
t = Timer(30.0, hello)
t.start() # after 30 seconds, "hello, world" will be printed
- class threading.Timer(interval, function, args=None, kwargs=None)¶
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 isNone
(the default) then an empty dict will be used.Alterado na versão 3.3: changed from a factory function to a class.
- 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¶
Adicionado na versão 3.2.
This class provides a simple synchronization primitive for use by a fixed number
of threads that need to wait for each other. Each of the threads tries to pass
the barrier by calling the wait()
method and will block until
all of the threads have made their 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)
- class threading.Barrier(parties, action=None, timeout=None)¶
Create a barrier object for parties number of threads. An action, when provided, is a callable to be called by one of the threads when they are released. timeout is the default timeout value if none is specified for the
wait()
method.- wait(timeout=None)¶
Pass the barrier. When all the threads party to the barrier have called this function, they are all released simultaneously. If a timeout is provided, 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
BrokenBarrierError
exception if the barrier is broken or reset while a thread is waiting.
- reset()¶
Return the barrier to the default, empty state. Any threads waiting on it will receive the
BrokenBarrierError
exception.Note that using this function may require some external synchronization if there are other threads whose state is unknown. If a barrier is broken it may be better to just leave it and create a new one.
- abort()¶
Put the barrier into a broken state. This causes any active or future calls to
wait()
to fail with theBrokenBarrierError
. Use this for example if one of the threads needs to abort, to avoid deadlocking the application.It may be preferable to simply create the barrier with a sensible timeout value to automatically guard against one of the threads going awry.
- parties¶
The number of threads required to pass the barrier.
- n_waiting¶
The number of threads currently waiting in the barrier.
- broken¶
A boolean that is
True
if the barrier is in the broken state.
- exception threading.BrokenBarrierError¶
This exception, a subclass of
RuntimeError
, is raised when theBarrier
object is reset or broken.
Using locks, conditions, and semaphores in the with
statement¶
All of the objects provided by this module that have acquire
and
release
methods can be used as context managers for a with
statement. The acquire
method will be called when the block is
entered, and release
will be called when the block is exited. Hence,
the following snippet:
with some_lock:
# do something...
é equivalente a:
some_lock.acquire()
try:
# do something...
finally:
some_lock.release()
Currently, Lock
, RLock
, Condition
,
Semaphore
, and BoundedSemaphore
objects may be used as
with
statement context managers.