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:mod:`threading` --- Thread-based parallelism
=============================================
.. module:: threading
:synopsis: Thread-based parallelism.
**Source code:** :source:`Lib/threading.py`
--------------
This module constructs higher-level threading interfaces on top of the lower
level :mod:`_thread` module. See also the :mod:`queue` module.
.. versionchanged:: 3.7
This module used to be optional, it is now always available.
.. note::
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.
.. impl-detail::
In CPython, due to the :term:`Global Interpreter Lock
<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:
.. 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`.
The function ``activeCount`` is a deprecated alias for this function.
.. 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.
The function ``currentThread`` is a deprecated alias for this function.
.. function:: excepthook(args, /)
Handle uncaught exception raised by :func:`Thread.run`.
The *args* argument has the following attributes:
* *exc_type*: Exception type.
* *exc_value*: Exception value, can be ``None``.
* *exc_traceback*: Exception traceback, can be ``None``.
* *thread*: Thread which raised the exception, can be ``None``.
If *exc_type* is :exc:`SystemExit`, the exception is silently ignored.
Otherwise, the exception is printed out on :data:`sys.stderr`.
If this function raises an exception, :func:`sys.excepthook` is called to
handle it.
:func:`threading.excepthook` can be overridden to control how uncaught
exceptions raised by :func:`Thread.run` are handled.
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.
.. seealso::
:func:`sys.excepthook` handles uncaught exceptions.
.. versionadded:: 3.8
.. data:: __excepthook__
Holds the original value of :func:`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.
.. versionadded:: 3.10
.. function:: get_ident()
Return the 'thread identifier' of the current thread. This is a nonzero
integer. Its value has no direct meaning; it is intended as a magic cookie
to be used e.g. to index a dictionary of thread-specific data. Thread
identifiers may be recycled when a thread exits and another thread is
created.
.. versionadded:: 3.3
.. function:: get_native_id()
Return the native integral Thread ID of the current thread assigned by the kernel.
This is a non-negative integer.
Its 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).
.. availability:: Windows, FreeBSD, Linux, macOS, OpenBSD, NetBSD, AIX.
.. versionadded:: 3.8
.. function:: enumerate()
Return a list of all :class:`Thread` objects currently active. The list
includes daemonic threads and dummy thread objects created by
:func:`current_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.
.. function:: main_thread()
Return the main :class:`Thread` object. In normal conditions, the
main thread is the thread from which the Python interpreter was
started.
.. versionadded:: 3.4
.. function:: settrace(func)
.. index:: single: trace function
Set a trace function for all threads started from the :mod:`threading` module.
The *func* will be passed to :func:`sys.settrace` for each thread, before its
:meth:`~Thread.run` method is called.
.. function:: gettrace()
.. index::
single: trace function
single: debugger
Get the trace function as set by :func:`settrace`.
.. versionadded:: 3.10
.. function:: setprofile(func)
.. index:: single: profile function
Set a profile function for all threads started from the :mod:`threading` module.
The *func* will be passed to :func:`sys.setprofile` for each thread, before its
:meth:`~Thread.run` method is called.
.. function:: getprofile()
.. index:: single: profile function
Get the profiler function as set by :func:`setprofile`.
.. versionadded:: 3.10
.. function:: stack_size([size])
Return the thread stack size used when creating new threads. The optional
*size* argument specifies the stack size to be used for subsequently created
threads, and must be 0 (use platform or configured default) or a positive
integer value of at least 32,768 (32 KiB). If *size* is not specified,
0 is used. If changing the thread stack size is
unsupported, a :exc:`RuntimeError` is raised. If the specified stack size is
invalid, a :exc:`ValueError` is raised and the stack size is unmodified. 32 KiB
is currently the minimum supported stack size value to guarantee sufficient
stack space for the interpreter itself. Note that some platforms may have
particular restrictions on values for the stack size, such as requiring a
minimum stack size > 32 KiB or requiring allocation in multiples of the system
memory page size - platform documentation should be referred to for more
information (4 KiB pages are common; using multiples of 4096 for the stack size is
the suggested approach in the absence of more specific information).
.. availability:: Windows, systems with POSIX threads.
This module also defines the following constant:
.. data:: TIMEOUT_MAX
The maximum value allowed for the *timeout* parameter of blocking functions
(:meth:`Lock.acquire`, :meth:`RLock.acquire`, :meth:`Condition.wait`, etc.).
Specifying a timeout greater than this value will raise an
:exc:`OverflowError`.
.. versionadded:: 3.2
This module defines a number of classes, which are detailed in the sections
below.
The design of this module is loosely based on Java's threading model. However,
where Java makes locks and condition variables basic behavior of every object,
they are separate objects in Python. Python's :class:`Thread` class supports a
subset of the behavior of Java's Thread class; currently, there are no
priorities, no thread groups, and threads cannot be destroyed, stopped,
suspended, resumed, or interrupted. The static methods of Java's Thread class,
when implemented, are mapped to module-level functions.
All of the methods described below are executed atomically.
Thread-Local Data
-----------------
Thread-local data is data whose values are thread specific. To manage
thread-local data, just create an instance of :class:`local` (or a
subclass) and store attributes on it::
mydata = threading.local()
mydata.x = 1
The instance's values will be different for separate threads.
.. class:: local()
A class that represents thread-local data.
For more details and extensive examples, see the documentation string of the
:mod:`_threading_local` module.
.. _thread-objects:
Thread Objects
--------------
The :class:`Thread` class represents an activity that is run in a separate
thread of control. There are two ways to specify the activity: by passing a
callable object to the constructor, or by overriding the :meth:`~Thread.run`
method in a subclass. No other methods (except for the constructor) should be
overridden in a subclass. In other words, *only* override the
:meth:`~Thread.__init__` and :meth:`~Thread.run` methods of this class.
Once a thread object is created, its activity must be started by calling the
thread's :meth:`~Thread.start` method. This invokes the :meth:`~Thread.run`
method in a separate thread of control.
Once the thread's activity is started, the thread is considered 'alive'. It
stops being alive when its :meth:`~Thread.run` method terminates -- either
normally, or by raising an unhandled exception. The :meth:`~Thread.is_alive`
method tests whether the thread is alive.
Other threads can call a thread's :meth:`~Thread.join` method. This blocks
the calling thread until the thread whose :meth:`~Thread.join` method is
called is terminated.
A thread has a name. The name can be passed to the constructor, and read or
changed through the :attr:`~Thread.name` attribute.
If the :meth:`~Thread.run` method raises an exception,
:func:`threading.excepthook` is called to handle it. By default,
:func:`threading.excepthook` ignores silently :exc:`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 :attr:`~Thread.daemon` property or the *daemon* constructor
argument.
.. note::
Daemon threads are abruptly stopped at shutdown. Their resources (such
as open files, database transactions, etc.) may not be released properly.
If you want your threads to stop gracefully, make them non-daemonic and
use a suitable signalling mechanism such as an :class:`Event`.
There is a "main thread" object; this corresponds to the initial thread of
control in the Python program. It is not a daemon thread.
There is the possibility that "dummy thread objects" are created. These are
thread objects corresponding to "alien threads", which are threads of control
started outside the threading module, such as directly from C code. Dummy
thread objects have limited functionality; they are always considered alive and
daemonic, and cannot be :meth:`~Thread.join`\ ed. They are never deleted,
since it is impossible to detect the termination of alien threads.
.. class:: 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 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,
or "Thread-*N* (target)" where "target" is ``target.__name__`` if the
*target* argument is specified.
*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 not ``None``, *daemon* explicitly sets whether the thread is daemonic.
If ``None`` (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.
.. versionchanged:: 3.10
Use the *target* name if *name* argument is omitted.
.. versionchanged:: 3.3
Added the *daemon* argument.
.. 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 positional 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()
Deprecated getter/setter API for :attr:`~Thread.name`; use it directly as a
property instead.
.. deprecated:: 3.10
.. 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:`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.
.. attribute:: native_id
The Thread ID (``TID``) of this thread, as assigned by the OS (kernel).
This is a non-negative integer, or ``None`` if the thread has not
been started. See the :func:`get_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).
.. note::
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:: Requires :func:`get_native_id` function.
.. versionadded:: 3.8
.. 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()
Deprecated getter/setter API for :attr:`~Thread.daemon`; use it directly as a
property instead.
.. deprecated:: 3.10
.. _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 management protocol <with-locks>`.
When more than one thread is blocked in :meth:`~Lock.acquire` waiting for the
state to turn to unlocked, only one thread proceeds when a :meth:`~Lock.release`
call resets the state to unlocked; which one of the waiting threads proceeds
is not defined, and may vary across implementations.
All methods are executed atomically.
.. class:: 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.
.. method:: acquire(blocking=True, timeout=-1)
Acquire a lock, blocking or non-blocking.
When invoked with the *blocking* argument set to ``True`` (the default),
block until the lock is unlocked, then set it to locked and return ``True``.
When invoked with the *blocking* argument set to ``False``, do not block.
If a call with *blocking* set to ``True`` would block, return ``False``
immediately; otherwise, set the lock to locked and return ``True``.
When invoked with the floating-point *timeout* argument set to a positive
value, block for at most the number of seconds specified by *timeout*
and as long as the lock cannot be acquired. A *timeout* argument of ``-1``
specifies an unbounded wait. It is forbidden to specify a *timeout*
when *blocking* is false.
The return value is ``True`` if the lock is acquired successfully,
``False`` if not (for example if the *timeout* expired).
.. versionchanged:: 3.2
The *timeout* parameter is new.
.. versionchanged:: 3.2
Lock acquisition can now be interrupted by signals on POSIX if the
underlying threading implementation supports it.
.. method:: release()
Release a lock. This can be called from any thread, not only the thread
which has acquired the lock.
When the lock is locked, reset it to unlocked, and return. If any other threads
are blocked waiting for the lock to become unlocked, allow exactly one of them
to proceed.
When invoked on an unlocked lock, a :exc:`RuntimeError` is raised.
There is no return value.
.. method:: locked()
Return true if the lock is acquired.
.. _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 management protocol <with-locks>`.
.. class:: 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.
.. method:: acquire(blocking=True, timeout=-1)
Acquire a lock, blocking or non-blocking.
When invoked without arguments: if this thread already owns the lock, increment
the recursion level by one, and return immediately. Otherwise, if another
thread owns the lock, block until the lock is unlocked. Once the lock is
unlocked (not owned by any thread), then grab ownership, set the recursion level
to one, and return. If more than one thread is blocked waiting until the lock
is unlocked, only one at a time will be able to grab ownership of the lock.
There is no return value in this case.
When invoked with the *blocking* argument set to true, do the same thing as when
called without arguments, and return ``True``.
When invoked with the *blocking* argument set to false, do not block. If a call
without an argument would block, return ``False`` immediately; otherwise, do the
same thing as when called without arguments, and return ``True``.
When invoked with the floating-point *timeout* argument set to a positive
value, block for at most the number of seconds specified by *timeout*
and as long as the lock cannot be acquired. Return ``True`` if the lock has
been acquired, false if the timeout has elapsed.
.. versionchanged:: 3.2
The *timeout* parameter is new.
.. method:: release()
Release a lock, decrementing the recursion level. If after the decrement it is
zero, reset the lock to unlocked (not owned by any thread), and if any other
threads are blocked waiting for the lock to become unlocked, allow exactly one
of them to proceed. If after the decrement the recursion level is still
nonzero, the lock remains locked and owned by the calling thread.
Only call this method when the calling thread owns the lock. A
:exc:`RuntimeError` is raised if this method is called when the lock is
unlocked.
There is no return value.
.. _condition-objects:
Condition Objects
-----------------
A condition variable is always associated with some kind of lock; this can be
passed in or one will be created by default. Passing one in is useful when
several condition variables must share the same lock. The lock is part of
the condition object: you don't have to track it separately.
A condition variable obeys the :ref:`context management protocol <with-locks>`:
using the ``with`` statement acquires the associated lock for the duration of
the enclosed block. The :meth:`~Condition.acquire` and
:meth:`~Condition.release` methods also call the corresponding methods of
the associated lock.
Other methods must be called with the associated lock held. The
:meth:`~Condition.wait` method releases the lock, and then blocks until
another thread awakens it by calling :meth:`~Condition.notify` or
:meth:`~Condition.notify_all`. Once awakened, :meth:`~Condition.wait`
re-acquires the lock and returns. It is also possible to specify a timeout.
The :meth:`~Condition.notify` method wakes up one of the threads waiting for
the condition variable, if any are waiting. The :meth:`~Condition.notify_all`
method wakes up all threads waiting for the condition variable.
Note: the :meth:`~Condition.notify` and :meth:`~Condition.notify_all` methods
don't release the lock; this means that the thread or threads awakened will
not return from their :meth:`~Condition.wait` call immediately, but only when
the thread that called :meth:`~Condition.notify` or :meth:`~Condition.notify_all`
finally relinquishes ownership of the lock.
The typical programming style using condition variables uses the lock to
synchronize access to some shared state; threads that are interested in a
particular change of state call :meth:`~Condition.wait` repeatedly until they
see the desired state, while threads that modify the state call
:meth:`~Condition.notify` or :meth:`~Condition.notify_all` when they change
the state in such a way that it could possibly be a desired state for one
of the waiters. For example, the following code is a generic
producer-consumer situation with unlimited buffer capacity::
# Consume one item
with cv:
while not an_item_is_available():
cv.wait()
get_an_available_item()
# Produce one item
with cv:
make_an_item_available()
cv.notify()
The ``while`` loop checking for the application's condition is necessary
because :meth:`~Condition.wait` can return after an arbitrary long time,
and the condition which prompted the :meth:`~Condition.notify` call may
no longer hold true. This is inherent to multi-threaded programming. The
:meth:`~Condition.wait_for` method can be used to automate the condition
checking, and eases the computation of timeouts::
# Consume an item
with cv:
cv.wait_for(an_item_is_available)
get_an_available_item()
To choose between :meth:`~Condition.notify` and :meth:`~Condition.notify_all`,
consider whether one state change can be interesting for only one or several
waiting threads. E.g. in a typical producer-consumer situation, adding one
item to the buffer only needs to wake up one consumer thread.
.. class:: 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 a :class:`Lock`
or :class:`RLock` object, and it is used as the underlying lock. Otherwise,
a new :class:`RLock` object is created and used as the underlying lock.
.. versionchanged:: 3.3
changed from a factory function to a class.
.. method:: acquire(*args)
Acquire the underlying lock. This method calls the corresponding method on
the underlying lock; the return value is whatever that method returns.
.. method:: release()
Release the underlying lock. This method calls the corresponding method on
the underlying lock; there is no return value.
.. method:: wait(timeout=None)
Wait until notified or until a timeout occurs. If the calling thread has
not acquired the lock when this method is called, a :exc:`RuntimeError` is
raised.
This method releases the underlying lock, and then blocks until it is
awakened by a :meth:`notify` or :meth:`notify_all` call for the same
condition variable in another thread, or until the optional timeout
occurs. Once awakened or timed out, it re-acquires the lock and returns.
When the *timeout* argument is present and not ``None``, it should be a
floating point number specifying a timeout for the operation in seconds
(or fractions thereof).
When the underlying lock is an :class:`RLock`, it is not released using
its :meth:`release` method, since this may not actually unlock the lock
when it was acquired multiple times recursively. Instead, an internal
interface of the :class:`RLock` class is used, which really unlocks it
even when it has been recursively acquired several times. Another internal
interface is then used to restore the recursion level when the lock is
reacquired.
The return value is ``True`` unless a given *timeout* expired, in which
case it is ``False``.
.. versionchanged:: 3.2
Previously, the method always returned ``None``.
.. method:: wait_for(predicate, timeout=None)
Wait until a condition evaluates to true. *predicate* should be a
callable which result will be interpreted as a boolean value.
A *timeout* may be provided giving the maximum time to wait.
This utility method may call :meth:`wait` repeatedly until the predicate
is satisfied, or until a timeout occurs. The return value is
the last return value of the predicate and will evaluate to
``False`` if the method timed out.
Ignoring the timeout feature, calling this method is roughly equivalent to
writing::
while not predicate():
cv.wait()
Therefore, the same rules apply as with :meth:`wait`: The lock must be
held when called and is re-acquired on return. The predicate is evaluated
with the lock held.
.. versionadded:: 3.2
.. method:: notify(n=1)
By default, wake up one thread waiting on this condition, if any. If the
calling thread has not acquired the lock when this method is called, a
:exc:`RuntimeError` is raised.
This method wakes up at most *n* of the threads waiting for the condition
variable; it is a no-op if no threads are waiting.
The current implementation wakes up exactly *n* threads, if at least *n*
threads are waiting. However, it's not safe to rely on this behavior.
A future, optimized implementation may occasionally wake up more than
*n* threads.
Note: an awakened thread does not actually return from its :meth:`wait`
call until it can reacquire the lock. Since :meth:`notify` does not
release the lock, its caller should.
.. method:: notify_all()
Wake up all threads waiting on this condition. This method acts like
:meth:`notify`, but wakes up all waiting threads instead of one. If the
calling thread has not acquired the lock when this method is called, a
:exc:`RuntimeError` is raised.
The method ``notifyAll`` is a deprecated alias for this method.
.. _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 management protocol <with-locks>`.
.. class:: Semaphore(value=1)
This class implements semaphore objects. A semaphore manages an atomic
counter representing the number of :meth:`release` calls minus the number of
:meth:`acquire` calls, plus an initial value. The :meth:`acquire` method
blocks if necessary until it can return without making the counter negative.
If not given, *value* defaults to 1.
The optional argument gives the initial *value* for the internal counter; it
defaults to ``1``. If the *value* given is less than 0, :exc:`ValueError` is
raised.
.. versionchanged:: 3.3
changed from a factory function to a class.
.. method:: acquire(blocking=True, timeout=None)
Acquire a semaphore.
When invoked without arguments:
* If the internal counter is larger than zero on entry, decrement it by
one and return ``True`` immediately.
* If the internal counter is zero on entry, block until awoken by a call to
:meth:`~Semaphore.release`. Once awoken (and the counter is greater
than 0), decrement the counter by 1 and return ``True``. Exactly one
thread will be awoken by each call to :meth:`~Semaphore.release`. The
order in which threads are awoken should not be relied on.
When invoked with *blocking* set to false, do not block. If a call
without an argument would block, return ``False`` immediately; otherwise, do
the same thing as when called without arguments, and return ``True``.
When invoked with a *timeout* other than ``None``, it will block for at
most *timeout* seconds. If acquire does not complete successfully in
that interval, return ``False``. Return ``True`` otherwise.
.. versionchanged:: 3.2
The *timeout* parameter is new.
.. method:: release(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.
.. versionchanged:: 3.9
Added the *n* parameter to release multiple waiting threads at once.
.. class:: 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,
:exc:`ValueError` is raised. In most situations semaphores are used to guard
resources with limited capacity. If the semaphore is released too many times
it's a sign of a bug. If not given, *value* defaults to 1.
.. versionchanged:: 3.3
changed from a factory function to a class.
.. _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()
Class implementing event objects. An event manages a flag that can be set to
true with the :meth:`~Event.set` method and reset to false with the
:meth:`clear` method. The :meth:`wait` method blocks until the flag is true.
The flag is initially false.
.. versionchanged:: 3.3
changed from a factory function to a class.
.. method:: is_set()
Return ``True`` if and only if the internal flag is true.
The method ``isSet`` is a deprecated alias for this method.
.. 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:`~Timer.start`
method. The timer can be stopped (before its action has begun) by calling the
:meth:`~Timer.cancel` method. The interval the timer will wait before
executing its action may not be exactly the same as the interval specified by
the user.
For example::
def hello():
print("hello, world")
t = Timer(30.0, hello)
t.start() # after 30 seconds, "hello, world" will be printed
.. class:: 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* is ``None`` (the default) then an empty dict will be used.
.. versionchanged:: 3.3
changed from a factory function to a class.
.. method:: cancel()
Stop the timer, and cancel the execution of the timer's action. This will
only work if the timer is still in its waiting stage.
Barrier Objects
---------------
.. versionadded:: 3.2
This class provides a simple synchronization primitive for use by a fixed number
of threads that need to wait for each other. Each of the threads tries to pass
the barrier by calling the :meth:`~Barrier.wait` method and will block until
all of the threads have made their :meth:`~Barrier.wait` calls. At this point,
the threads are released simultaneously.
The barrier can be reused any number of times for the same number of threads.
As an example, here is a simple way to synchronize a client and server thread::
b = Barrier(2, timeout=5)
def server():
start_server()
b.wait()
while True:
connection = accept_connection()
process_server_connection(connection)
def client():
b.wait()
while True:
connection = make_connection()
process_client_connection(connection)
.. 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 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 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.
.. 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.
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