cpython/Doc/library/threading.rst

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:mod:`threading` --- Higher-level threading interface
=====================================================
.. module:: threading
:synopsis: Higher-level threading interface.
This module constructs higher-level threading interfaces on top of the lower
level :mod:`_thread` module. See also the :mod:`queue` module.
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The :mod:`dummy_threading` module is provided for situations where
:mod:`threading` cannot be used because :mod:`_thread` is missing.
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.. note::
While they are not listed below, the ``camelCase`` names used for some
methods and functions in this module in the Python 2.x series are still
supported by this module.
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This module defines the following functions and objects:
.. function:: active_count()
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Return the number of :class:`Thread` objects currently alive. The returned
count is equal to the length of the list returned by :func:`enumerate`.
.. function:: Condition()
:noindex:
A factory function that returns a new condition variable object. A condition
variable allows one or more threads to wait until they are notified by another
thread.
.. function:: current_thread()
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Return the current :class:`Thread` object, corresponding to the caller's thread
of control. If the caller's thread of control was not created through the
:mod:`threading` module, a dummy thread object with limited functionality is
returned.
.. function:: enumerate()
Return a list of all :class:`Thread` objects currently alive. The list
includes daemonic threads, dummy thread objects created by
:func:`current_thread`, and the main thread. It excludes terminated threads
and threads that have not yet been started.
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.. function:: Event()
:noindex:
A factory function that returns a new event object. An event manages a flag
that can be set to true with the :meth:`~Event.set` method and reset to false
with the :meth:`clear` method. The :meth:`wait` method blocks until the flag
is true.
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.. class:: local
A class that represents thread-local data. Thread-local data are data whose
values are thread specific. To manage thread-local data, just create an
instance of :class:`local` (or a subclass) and store attributes on it::
mydata = threading.local()
mydata.x = 1
The instance's values will be different for separate threads.
For more details and extensive examples, see the documentation string of the
:mod:`_threading_local` module.
.. function:: Lock()
A factory function that returns a new primitive lock object. Once a thread has
acquired it, subsequent attempts to acquire it block, until it is released; any
thread may release it.
.. function:: RLock()
A factory function that returns a new reentrant lock object. A reentrant lock
must be released by the thread that acquired it. Once a thread has acquired a
reentrant lock, the same thread may acquire it again without blocking; the
thread must release it once for each time it has acquired it.
.. function:: Semaphore([value])
:noindex:
A factory function that returns a new semaphore object. A semaphore manages a
counter representing the number of :meth:`release` calls minus the number of
:meth:`acquire` calls, plus an initial value. The :meth:`acquire` method blocks
if necessary until it can return without making the counter negative. If not
given, *value* defaults to 1.
.. function:: BoundedSemaphore([value])
A factory function that returns a new bounded semaphore object. A bounded
semaphore checks to make sure its current value doesn't exceed its initial
value. If it does, :exc:`ValueError` is raised. In most situations semaphores
are used to guard resources with limited capacity. If the semaphore is released
too many times it's a sign of a bug. If not given, *value* defaults to 1.
.. class:: Thread
A class that represents a thread of control. This class can be safely
subclassed in a limited fashion.
.. class:: Timer
A thread that executes a function after a specified interval has passed.
.. function:: settrace(func)
.. index:: single: trace function
Set a trace function for all threads started from the :mod:`threading` module.
The *func* will be passed to :func:`sys.settrace` for each thread, before its
:meth:`run` method is called.
.. function:: setprofile(func)
.. index:: single: profile function
Set a profile function for all threads started from the :mod:`threading` module.
The *func* will be passed to :func:`sys.setprofile` for each thread, before its
:meth:`run` method is called.
.. function:: stack_size([size])
Return the thread stack size used when creating new threads. The optional
*size* argument specifies the stack size to be used for subsequently created
threads, and must be 0 (use platform or configured default) or a positive
integer value of at least 32,768 (32kB). If changing the thread stack size is
unsupported, a :exc:`ThreadError` is raised. If the specified stack size is
invalid, a :exc:`ValueError` is raised and the stack size is unmodified. 32kB
is currently the minimum supported stack size value to guarantee sufficient
stack space for the interpreter itself. Note that some platforms may have
particular restrictions on values for the stack size, such as requiring a
minimum stack size > 32kB or requiring allocation in multiples of the system
memory page size - platform documentation should be referred to for more
information (4kB pages are common; using multiples of 4096 for the stack size is
the suggested approach in the absence of more specific information).
Availability: Windows, systems with POSIX threads.
Detailed interfaces for the objects are documented below.
The design of this module is loosely based on Java's threading model. However,
where Java makes locks and condition variables basic behavior of every object,
they are separate objects in Python. Python's :class:`Thread` class supports a
subset of the behavior of Java's Thread class; currently, there are no
priorities, no thread groups, and threads cannot be destroyed, stopped,
suspended, resumed, or interrupted. The static methods of Java's Thread class,
when implemented, are mapped to module-level functions.
All of the methods described below are executed atomically.
.. _thread-objects:
Thread Objects
--------------
This class represents an activity that is run in a separate thread of control.
There are two ways to specify the activity: by passing a callable object to the
constructor, or by overriding the :meth:`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
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through the :attr:`daemon` property.
There is a "main thread" object; this corresponds to the initial thread of
control in the Python program. It is not a daemon thread.
There is the possibility that "dummy thread objects" are created. These are
thread objects corresponding to "alien threads", which are threads of control
started outside the threading module, such as directly from C code. Dummy
thread objects have limited functionality; they are always considered alive and
daemonic, and cannot be :meth:`join`\ ed. They are never deleted, since it is
impossible to detect the termination of alien threads.
.. class:: Thread(group=None, target=None, name=None, args=(), kwargs={})
This constructor should always be called with keyword arguments. Arguments
are:
*group* should be ``None``; reserved for future extension when a
:class:`ThreadGroup` class is implemented.
*target* is the callable object to be invoked by the :meth:`run` method.
Defaults to ``None``, meaning nothing is called.
*name* is the thread name. By default, a unique name is constructed of the
form "Thread-*N*" where *N* is a small decimal number.
*args* is the argument tuple for the target invocation. Defaults to ``()``.
*kwargs* is a dictionary of keyword arguments for the target invocation.
Defaults to ``{}``.
If the subclass overrides the constructor, it must make sure to invoke the
base class constructor (``Thread.__init__()``) before doing anything else to
the thread.
.. method:: start()
Start the thread's activity.
It must be called at most once per thread object. It arranges for the
object's :meth:`run` method to be invoked in a separate thread of control.
This method will raise a :exc:`RuntimeException` if called more than once
on the same thread object.
.. method:: run()
Method representing the thread's activity.
You may override this method in a subclass. The standard :meth:`run`
method invokes the callable object passed to the object's constructor as
the *target* argument, if any, with sequential and keyword arguments taken
from the *args* and *kwargs* arguments, respectively.
.. method:: join([timeout])
Wait until the thread terminates. This blocks the calling thread until the
thread whose :meth:`join` method is called terminates -- either normally
or through an unhandled exception -- or until the optional timeout occurs.
When the *timeout* argument is present and not ``None``, it should be a
floating point number specifying a timeout for the operation in seconds
(or fractions thereof). As :meth:`join` always returns ``None``, you must
call :meth:`is_alive` after :meth:`join` to decide whether a timeout
happened -- if the thread is still alive, the :meth:`join` call timed out.
When the *timeout* argument is not present or ``None``, the operation will
block until the thread terminates.
A thread can be :meth:`join`\ ed many times.
:meth:`join` raises a :exc:`RuntimeError` if an attempt is made to join
the current thread as that would cause a deadlock. It is also an error to
:meth:`join` a thread before it has been started and attempts to do so
raises the same exception.
.. attribute:: name
A string used for identification purposes only. It has no semantics.
Multiple threads may be given the same name. The initial name is set by
the constructor.
.. method:: getName()
setName()
Old getter/setter API for :attr:`~Thread.name`; use it directly as a
property instead.
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.. attribute:: ident
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The 'thread identifier' of this thread or ``None`` if the thread has not
been started. This is a nonzero integer. See the
:func:`thread.get_ident()` function. Thread identifiers may be recycled
when a thread exits and another thread is created. The identifier is
available even after the thread has exited.
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.. method:: is_alive()
Return whether the thread is alive.
Roughly, a thread is alive from the moment the :meth:`start` method
returns until its :meth:`run` method terminates. The module function
:func:`enumerate` returns a list of all alive threads.
.. attribute:: daemon
A boolean value indicating whether this thread is a daemon thread (True)
or not (False). This must be set before :meth:`start` is called,
otherwise :exc:`RuntimeError` is raised. Its initial value is inherited
from the creating thread; the main thread is not a daemon thread and
therefore all threads created in the main thread default to :attr:`daemon`
= ``False``.
The entire Python program exits when no alive non-daemon threads are left.
.. method:: isDaemon()
setDaemon()
Old getter/setter API for :attr:`~Thread.daemon`; use it directly as a
property instead.
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.. _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`
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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.
.. method:: Lock.acquire([blocking=1])
Acquire a lock, blocking or non-blocking.
When invoked without arguments, block until the lock is unlocked, then set it to
locked, and return true.
When invoked with the *blocking* argument set to true, do the same thing as when
called without arguments, and return true.
When invoked with the *blocking* argument set to false, do not block. If a call
without an argument would block, return false immediately; otherwise, do the
same thing as when called without arguments, and return true.
.. method:: Lock.release()
Release a lock.
When the lock is locked, reset it to unlocked, and return. If any other threads
are blocked waiting for the lock to become unlocked, allow exactly one of them
to proceed.
Do not call this method when the lock is unlocked.
There is no return value.
.. _rlock-objects:
RLock Objects
-------------
A reentrant lock is a synchronization primitive that may be acquired multiple
times by the same thread. Internally, it uses the concepts of "owning thread"
and "recursion level" in addition to the locked/unlocked state used by primitive
locks. In the locked state, some thread owns the lock; in the unlocked state,
no thread owns it.
To lock the lock, a thread calls its :meth:`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.
.. method:: RLock.acquire([blocking=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.
.. method:: RLock.release()
Release a lock, decrementing the recursion level. If after the decrement it is
zero, reset the lock to unlocked (not owned by any thread), and if any other
threads are blocked waiting for the lock to become unlocked, allow exactly one
of them to proceed. If after the decrement the recursion level is still
nonzero, the lock remains locked and owned by the calling thread.
Only call this method when the calling thread owns the lock. A
:exc:`RuntimeError` is raised if this method is called when the lock is
unlocked.
There is no return value.
.. _condition-objects:
Condition Objects
-----------------
A condition variable is always associated with some kind of lock; this can be
passed in or one will be created by default. (Passing one in is useful when
several condition variables must share the same lock.)
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`
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method, and :meth:`notify` and :meth:`notify_all` methods. These three must only
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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
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by a :meth:`notify` or :meth:`notify_all` call for the same condition variable in
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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
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variable, if any are waiting. The :meth:`notify_all` method wakes up all threads
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waiting for the condition variable.
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Note: the :meth:`notify` and :meth:`notify_all` methods don't release the lock;
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this means that the thread or threads awakened will not return from their
:meth:`wait` call immediately, but only when the thread that called
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:meth:`notify` or :meth:`notify_all` finally relinquishes ownership of the lock.
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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
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:meth:`notify_all` when they change the state in such a way that it could
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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
cv.acquire()
while not an_item_is_available():
cv.wait()
get_an_available_item()
cv.release()
# Produce one item
cv.acquire()
make_an_item_available()
cv.notify()
cv.release()
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To choose between :meth:`notify` and :meth:`notify_all`, consider whether one
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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])
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.
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.. method:: acquire(*args)
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Acquire the underlying lock. This method calls the corresponding method on
the underlying lock; the return value is whatever that method returns.
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.. method:: release()
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Release the underlying lock. This method calls the corresponding method on
the underlying lock; there is no return value.
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.. method:: wait([timeout])
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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.
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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.
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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).
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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.
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.. method:: notify()
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Wake up a 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.
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This method wakes up one of the threads waiting for the condition
variable, if any are waiting; it is a no-op if no threads are waiting.
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The current implementation wakes up exactly one thread, if any are
waiting. However, it's not safe to rely on this behavior. A future,
optimized implementation may occasionally wake up more than one thread.
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Note: the awakened thread does not actually return from its :meth:`wait`
call until it can reacquire the lock. Since :meth:`notify` does not
release the lock, its caller should.
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.. method:: notify_all()
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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.
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.. _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 :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`.
.. class:: Semaphore([value])
The optional argument gives the initial *value* for the internal counter; it
defaults to ``1``. If the *value* given is less than 0, :exc:`ValueError` is
raised.
.. method:: acquire([blocking])
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Acquire a semaphore.
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When invoked without arguments: if the internal counter is larger than
zero on entry, decrement it by one and return immediately. If it is zero
on entry, block, waiting until some other thread has called
:meth:`release` to make it larger than zero. This is done with proper
interlocking so that if multiple :meth:`acquire` calls are blocked,
:meth:`release` will wake exactly one of them up. The implementation may
pick one at random, so the order in which blocked threads are awakened
should not be relied on. There is no return value in this case.
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When invoked with *blocking* set to true, do the same thing as when called
without arguments, and return true.
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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.
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.. method:: release()
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Release a semaphore, incrementing the internal counter by one. When it
was zero on entry and another thread is waiting for it to become larger
than zero again, wake up that thread.
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.. _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 size 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::
pool_sema.acquire()
conn = connectdb()
... use connection ...
conn.close()
pool_sema.release()
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:`clear` method. The
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:meth:`wait` method blocks until the flag is true.
.. class:: Event()
The internal flag is initially false.
.. method:: is_set()
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Return true if and only if the internal flag is true.
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.. method:: set()
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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.
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.. method:: clear()
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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.
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.. method:: wait([timeout])
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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.
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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).
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This method returns the internal flag on exit, so it will always return
``True`` except if a timeout is given and the operation times out.
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.. versionchanged:: 3.1
Previously, the method always returned ``None``.
Merged revisions 70712,70714,70764-70765,70769-70771,70773,70776-70777,70788-70789,70824,70828,70832,70836,70842,70851,70855,70857,70866-70872,70883,70885,70893-70894,70896-70897,70903,70905-70907,70915,70927,70933,70951,70960,70962-70964,70998,71001,71006,71008,71010-71011,71019,71037,71056,71094,71101-71103,71106,71119,71123,71149-71150,71203,71212,71214-71217,71221,71240 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r70712 | benjamin.peterson | 2009-03-30 10:15:38 -0500 (Mon, 30 Mar 2009) | 1 line don't rely on the order dict repr #5605 ........ r70714 | brett.cannon | 2009-03-30 10:20:53 -0500 (Mon, 30 Mar 2009) | 1 line Add an entry to developers.txt. ........ r70764 | martin.v.loewis | 2009-03-30 17:06:33 -0500 (Mon, 30 Mar 2009) | 2 lines Add several VM developers. ........ r70765 | georg.brandl | 2009-03-30 17:09:34 -0500 (Mon, 30 Mar 2009) | 1 line #5199: make warning about vars() assignment more visible. ........ r70769 | andrew.kuchling | 2009-03-30 17:29:53 -0500 (Mon, 30 Mar 2009) | 1 line Remove comment ........ r70770 | andrew.kuchling | 2009-03-30 17:30:20 -0500 (Mon, 30 Mar 2009) | 1 line Add several items and placeholders ........ r70771 | andrew.kuchling | 2009-03-30 17:31:11 -0500 (Mon, 30 Mar 2009) | 1 line Many edits ........ r70773 | georg.brandl | 2009-03-30 17:43:00 -0500 (Mon, 30 Mar 2009) | 1 line #5039: make it clear that the impl. note refers to CPython. ........ r70776 | andrew.kuchling | 2009-03-30 18:08:24 -0500 (Mon, 30 Mar 2009) | 1 line typo fix ........ r70777 | andrew.kuchling | 2009-03-30 18:09:46 -0500 (Mon, 30 Mar 2009) | 1 line Add more items ........ r70788 | andrew.kuchling | 2009-03-30 20:21:01 -0500 (Mon, 30 Mar 2009) | 1 line Add various items ........ r70789 | georg.brandl | 2009-03-30 20:25:15 -0500 (Mon, 30 Mar 2009) | 1 line Fix a wrong struct field assignment (docstring as closure). ........ r70824 | georg.brandl | 2009-03-31 10:43:20 -0500 (Tue, 31 Mar 2009) | 1 line #5519: remove reference to Kodos, which seems dead. ........ r70828 | georg.brandl | 2009-03-31 10:50:16 -0500 (Tue, 31 Mar 2009) | 1 line #5581: fget argument of abstractproperty is optional as well. ........ r70832 | georg.brandl | 2009-03-31 11:31:11 -0500 (Tue, 31 Mar 2009) | 1 line #1386675: specify WindowsError as the exception, because it has a winerror attribute that EnvironmentError doesnt have. ........ r70836 | georg.brandl | 2009-03-31 11:50:25 -0500 (Tue, 31 Mar 2009) | 1 line #5417: replace references to undocumented functions by ones to documented functions. ........ r70842 | georg.brandl | 2009-03-31 12:13:06 -0500 (Tue, 31 Mar 2009) | 1 line #970783: document PyObject_Generic[GS]etAttr. ........ r70851 | georg.brandl | 2009-03-31 13:26:55 -0500 (Tue, 31 Mar 2009) | 1 line #837577: note cryptic return value of spawn*e on invalid env dicts. ........ r70855 | georg.brandl | 2009-03-31 13:30:37 -0500 (Tue, 31 Mar 2009) | 1 line #5245: note that PyRun_SimpleString doesnt return on SystemExit. ........ r70857 | georg.brandl | 2009-03-31 13:33:10 -0500 (Tue, 31 Mar 2009) | 1 line #5227: note that Py_Main doesnt return on SystemExit. ........ r70866 | georg.brandl | 2009-03-31 14:06:57 -0500 (Tue, 31 Mar 2009) | 1 line #4882: document named group behavior a bit better. ........ r70867 | georg.brandl | 2009-03-31 14:10:35 -0500 (Tue, 31 Mar 2009) | 1 line #1096310: document usage of sys.__std*__ a bit better. ........ r70868 | georg.brandl | 2009-03-31 14:12:17 -0500 (Tue, 31 Mar 2009) | 1 line #5190: export make_option in __all__. ........ r70869 | georg.brandl | 2009-03-31 14:14:42 -0500 (Tue, 31 Mar 2009) | 1 line Fix-up unwanted change. ........ r70870 | georg.brandl | 2009-03-31 14:26:24 -0500 (Tue, 31 Mar 2009) | 1 line #4411: document mro() and __mro__. (I hope I got it right.) ........ r70871 | georg.brandl | 2009-03-31 14:30:56 -0500 (Tue, 31 Mar 2009) | 1 line #5618: fix typo. ........ r70872 | r.david.murray | 2009-03-31 14:31:17 -0500 (Tue, 31 Mar 2009) | 3 lines Delete out-of-date and little-known README from the test directory by consensus of devs at pycon sprint. ........ r70883 | georg.brandl | 2009-03-31 15:41:08 -0500 (Tue, 31 Mar 2009) | 1 line #1674032: return value of flag from Event.wait(). OKed by Guido. ........ r70885 | tarek.ziade | 2009-03-31 15:48:31 -0500 (Tue, 31 Mar 2009) | 1 line using log.warn for sys.stderr ........ r70893 | georg.brandl | 2009-03-31 15:56:32 -0500 (Tue, 31 Mar 2009) | 1 line #1530012: move TQS section before raw strings. ........ r70894 | benjamin.peterson | 2009-03-31 16:06:30 -0500 (Tue, 31 Mar 2009) | 1 line take the usual lock precautions around _active_limbo_lock ........ r70896 | georg.brandl | 2009-03-31 16:15:33 -0500 (Tue, 31 Mar 2009) | 1 line #5598: document DocFileSuite *args argument. ........ r70897 | benjamin.peterson | 2009-03-31 16:34:42 -0500 (Tue, 31 Mar 2009) | 1 line fix Thread.ident when it is the main thread or a dummy thread #5632 ........ r70903 | georg.brandl | 2009-03-31 16:45:18 -0500 (Tue, 31 Mar 2009) | 1 line #1676135: remove trailing slashes from --prefix argument. ........ r70905 | georg.brandl | 2009-03-31 17:03:40 -0500 (Tue, 31 Mar 2009) | 1 line #5563: more documentation for bdist_msi. ........ r70906 | georg.brandl | 2009-03-31 17:11:53 -0500 (Tue, 31 Mar 2009) | 1 line #1651995: fix _convert_ref for non-ASCII characters. ........ r70907 | georg.brandl | 2009-03-31 17:18:19 -0500 (Tue, 31 Mar 2009) | 1 line #3427: document correct return type for urlopen().info(). ........ r70915 | georg.brandl | 2009-03-31 17:40:16 -0500 (Tue, 31 Mar 2009) | 1 line #5018: remove confusing paragraph. ........ r70927 | georg.brandl | 2009-03-31 18:01:27 -0500 (Tue, 31 Mar 2009) | 1 line Dont shout to users. ........ r70933 | georg.brandl | 2009-03-31 19:04:33 -0500 (Tue, 31 Mar 2009) | 2 lines Issue #5635: Fix running test_sys with tracing enabled. ........ r70951 | georg.brandl | 2009-04-01 09:02:27 -0500 (Wed, 01 Apr 2009) | 1 line Add Maksim, who worked on several issues at the sprint. ........ r70960 | jesse.noller | 2009-04-01 11:42:19 -0500 (Wed, 01 Apr 2009) | 1 line Issue 3270: document Listener address restrictions on windows ........ r70962 | brett.cannon | 2009-04-01 12:07:16 -0500 (Wed, 01 Apr 2009) | 2 lines Ron DuPlain was given commit privileges at PyCon 2009 to work on 3to2. ........ r70963 | georg.brandl | 2009-04-01 12:46:01 -0500 (Wed, 01 Apr 2009) | 1 line #5655: fix docstring oversight. ........ r70964 | brett.cannon | 2009-04-01 12:52:13 -0500 (Wed, 01 Apr 2009) | 2 lines Paul Kippes was given commit privileges to work on 3to2. ........ r70998 | georg.brandl | 2009-04-01 16:54:21 -0500 (Wed, 01 Apr 2009) | 1 line In Pdb, stop assigning values to __builtin__._ which interferes with the one commonly installed by gettext. ........ r71001 | brett.cannon | 2009-04-01 18:01:12 -0500 (Wed, 01 Apr 2009) | 3 lines Add my initials to Misc/developers.txt. Names are now sorted by number of characters in the person's name. ........ r71006 | georg.brandl | 2009-04-01 18:32:17 -0500 (Wed, 01 Apr 2009) | 1 line Cache the f_locals dict of the current frame, since every access to frame.f_locals overrides its contents with the real locals which undoes modifications made by the debugging user. ........ r71008 | andrew.kuchling | 2009-04-01 19:02:14 -0500 (Wed, 01 Apr 2009) | 1 line Typo fix ........ r71010 | benjamin.peterson | 2009-04-01 19:11:52 -0500 (Wed, 01 Apr 2009) | 1 line fix markup ........ r71011 | benjamin.peterson | 2009-04-01 19:12:47 -0500 (Wed, 01 Apr 2009) | 1 line this should be :noindex: ........ r71019 | georg.brandl | 2009-04-01 21:00:01 -0500 (Wed, 01 Apr 2009) | 1 line Fix test_doctest, missed two assignments to curframe. ........ r71037 | r.david.murray | 2009-04-01 23:34:04 -0500 (Wed, 01 Apr 2009) | 6 lines Clarify that datetime strftime does not produce leap seconds and datetime strptime does not accept it in the strftime behavior section of the datetime docs. Closes issue 2568. ........ r71056 | georg.brandl | 2009-04-02 12:43:07 -0500 (Thu, 02 Apr 2009) | 2 lines Actually the displayhook should print the repr. ........ r71094 | vinay.sajip | 2009-04-03 05:23:18 -0500 (Fri, 03 Apr 2009) | 1 line Added warning about logging use from asynchronous signal handlers. ........ r71101 | andrew.kuchling | 2009-04-03 16:43:00 -0500 (Fri, 03 Apr 2009) | 1 line Add some items ........ r71102 | andrew.kuchling | 2009-04-03 16:44:49 -0500 (Fri, 03 Apr 2009) | 1 line Fix 'the the'; grammar fix ........ r71103 | andrew.kuchling | 2009-04-03 16:45:29 -0500 (Fri, 03 Apr 2009) | 1 line Fix 'the the' duplication ........ r71106 | vinay.sajip | 2009-04-03 16:58:16 -0500 (Fri, 03 Apr 2009) | 1 line Clarified warning about logging use from asynchronous signal handlers. ........ r71119 | raymond.hettinger | 2009-04-04 00:37:47 -0500 (Sat, 04 Apr 2009) | 1 line Add helpful link. ........ r71123 | r.david.murray | 2009-04-04 01:39:56 -0500 (Sat, 04 Apr 2009) | 2 lines Fix error in description of 'oct' (issue 5678). ........ r71149 | georg.brandl | 2009-04-04 08:42:39 -0500 (Sat, 04 Apr 2009) | 1 line #5642: clarify map() compatibility to the builtin. ........ r71150 | georg.brandl | 2009-04-04 08:45:49 -0500 (Sat, 04 Apr 2009) | 1 line #5601: clarify that webbrowser is not meant for file names. ........ r71203 | benjamin.peterson | 2009-04-04 18:46:34 -0500 (Sat, 04 Apr 2009) | 1 line note how using iter* are unsafe while mutating and document iter(dict) ........ r71212 | georg.brandl | 2009-04-05 05:24:20 -0500 (Sun, 05 Apr 2009) | 1 line #1742837: expand HTTP server docs, and fix SocketServer ones to document methods as methods, not functions. ........ r71214 | georg.brandl | 2009-04-05 05:29:57 -0500 (Sun, 05 Apr 2009) | 1 line Normalize spelling of Mac OS X. ........ r71215 | georg.brandl | 2009-04-05 05:32:26 -0500 (Sun, 05 Apr 2009) | 1 line Avoid sure signs of a diseased mind. ........ r71216 | georg.brandl | 2009-04-05 05:41:02 -0500 (Sun, 05 Apr 2009) | 1 line #1718017: document the relation of os.path and the posixpath, ntpath etc. modules better. ........ r71217 | georg.brandl | 2009-04-05 05:48:47 -0500 (Sun, 05 Apr 2009) | 1 line #1726172: dont raise an unexpected IndexError if a voidresp() call has an empty response. ........ r71221 | vinay.sajip | 2009-04-05 06:06:24 -0500 (Sun, 05 Apr 2009) | 1 line Issue #5695: Moved logging.captureWarnings() call inside with statement in WarningsTest.test_warnings. ........ r71240 | georg.brandl | 2009-04-05 09:40:06 -0500 (Sun, 05 Apr 2009) | 1 line #5370: doc update about unpickling objects with custom __getattr__ etc. methods. ........
2009-04-05 16:13:16 -03:00
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.. _timer-objects:
Timer Objects
-------------
This class represents an action that should be run only after a certain amount
of time has passed --- a timer. :class:`Timer` is a subclass of :class:`Thread`
and as such also functions as an example of creating custom threads.
Timers are started, as with threads, by calling their :meth:`start` method. The
timer can be stopped (before its action has begun) by calling the :meth:`cancel`
method. The interval the timer will wait before executing its action may not be
exactly the same as the interval specified by the user.
For example::
def hello():
print("hello, world")
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t = Timer(30.0, hello)
t.start() # after 30 seconds, "hello, world" will be printed
.. class:: Timer(interval, function, args=[], kwargs={})
Create a timer that will run *function* with arguments *args* and keyword
arguments *kwargs*, after *interval* seconds have passed.
.. method:: cancel()
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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.
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.. _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.
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")
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Merged revisions 61239-61249,61252-61257,61260-61264,61269-61275,61278-61279,61285-61286,61288-61290,61298,61303-61305,61312-61314,61317,61329,61332,61344,61350-61351,61363-61376,61378-61379,61382-61383,61387-61388,61392,61395-61396,61402-61403 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r61239 | andrew.kuchling | 2008-03-05 01:44:41 +0100 (Wed, 05 Mar 2008) | 1 line Add more items; add fragmentary notes ........ r61240 | amaury.forgeotdarc | 2008-03-05 02:50:33 +0100 (Wed, 05 Mar 2008) | 13 lines Issue#2238: some syntax errors from *args or **kwargs expressions would give bogus error messages, because of untested exceptions:: >>> f(**g(1=2)) XXX undetected error Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'int' object is not iterable instead of the expected SyntaxError: keyword can't be an expression Will backport. ........ r61241 | neal.norwitz | 2008-03-05 06:10:48 +0100 (Wed, 05 Mar 2008) | 3 lines Remove the files/dirs after closing the DB so the tests work on Windows. Patch from Trent Nelson. Also simplified removing a file by using test_support. ........ r61242 | neal.norwitz | 2008-03-05 06:14:18 +0100 (Wed, 05 Mar 2008) | 3 lines Get this test to pass even when there is no sound card in the system. Patch from Trent Nelson. (I can't test this.) ........ r61243 | neal.norwitz | 2008-03-05 06:20:44 +0100 (Wed, 05 Mar 2008) | 3 lines Catch OSError when trying to remove a file in case removal fails. This should prevent a failure in tearDown masking any real test failure. ........ r61244 | neal.norwitz | 2008-03-05 06:38:06 +0100 (Wed, 05 Mar 2008) | 5 lines Make the timeout longer to give slow machines a chance to pass the test before timing out. This doesn't change the duration of the test under normal circumstances. This is targetted at fixing the spurious failures on the FreeBSD buildbot primarily. ........ r61245 | neal.norwitz | 2008-03-05 06:49:03 +0100 (Wed, 05 Mar 2008) | 1 line Tabs -> spaces ........ r61246 | neal.norwitz | 2008-03-05 06:50:20 +0100 (Wed, 05 Mar 2008) | 1 line Use -u urlfetch to run more tests ........ r61247 | neal.norwitz | 2008-03-05 06:51:20 +0100 (Wed, 05 Mar 2008) | 1 line test_smtplib sometimes reports leaks too, suppress it ........ r61248 | jeffrey.yasskin | 2008-03-05 07:19:56 +0100 (Wed, 05 Mar 2008) | 5 lines Fix test_socketserver on Windows after r61099 added several signal.alarm() calls (which don't exist on non-Unix platforms). Thanks to Trent Nelson for the report and patch. ........ r61249 | georg.brandl | 2008-03-05 08:10:35 +0100 (Wed, 05 Mar 2008) | 2 lines Fix some rst. ........ r61252 | thomas.heller | 2008-03-05 15:53:39 +0100 (Wed, 05 Mar 2008) | 2 lines News entry for yesterdays commit. ........ r61253 | thomas.heller | 2008-03-05 16:34:29 +0100 (Wed, 05 Mar 2008) | 3 lines Issue 1872: Changed the struct module typecode from 't' to '?', for compatibility with PEP3118. ........ r61254 | skip.montanaro | 2008-03-05 17:41:09 +0100 (Wed, 05 Mar 2008) | 4 lines Elaborate on the role of the altinstall target when installing multiple versions. ........ r61255 | georg.brandl | 2008-03-05 20:31:44 +0100 (Wed, 05 Mar 2008) | 2 lines #2239: PYTHONPATH delimiter is os.pathsep. ........ r61256 | raymond.hettinger | 2008-03-05 21:59:58 +0100 (Wed, 05 Mar 2008) | 1 line C implementation of itertools.permutations(). ........ r61257 | raymond.hettinger | 2008-03-05 22:04:32 +0100 (Wed, 05 Mar 2008) | 1 line Small code cleanup. ........ r61260 | martin.v.loewis | 2008-03-05 23:24:31 +0100 (Wed, 05 Mar 2008) | 2 lines cd PCbuild only after deleting all pyc files. ........ r61261 | raymond.hettinger | 2008-03-06 02:15:52 +0100 (Thu, 06 Mar 2008) | 1 line Add examples. ........ r61262 | andrew.kuchling | 2008-03-06 02:36:27 +0100 (Thu, 06 Mar 2008) | 1 line Add two items ........ r61263 | georg.brandl | 2008-03-06 07:47:18 +0100 (Thu, 06 Mar 2008) | 2 lines #1725737: ignore other VC directories other than CVS and SVN's too. ........ r61264 | martin.v.loewis | 2008-03-06 07:55:22 +0100 (Thu, 06 Mar 2008) | 4 lines Patch #2232: os.tmpfile might fail on Windows if the user has no permission to create files in the root directory. Will backport to 2.5. ........ r61269 | georg.brandl | 2008-03-06 08:19:15 +0100 (Thu, 06 Mar 2008) | 2 lines Expand on re.split behavior with captured expressions. ........ r61270 | georg.brandl | 2008-03-06 08:22:09 +0100 (Thu, 06 Mar 2008) | 2 lines Little clarification of assignments. ........ r61271 | georg.brandl | 2008-03-06 08:31:34 +0100 (Thu, 06 Mar 2008) | 2 lines Add isinstance/issubclass to tutorial. ........ r61272 | georg.brandl | 2008-03-06 08:34:52 +0100 (Thu, 06 Mar 2008) | 2 lines Add missing NEWS entry for r61263. ........ r61273 | georg.brandl | 2008-03-06 08:41:16 +0100 (Thu, 06 Mar 2008) | 2 lines #2225: return nonzero status code from py_compile if not all files could be compiled. ........ r61274 | georg.brandl | 2008-03-06 08:43:02 +0100 (Thu, 06 Mar 2008) | 2 lines #2220: handle matching failure more gracefully. ........ r61275 | georg.brandl | 2008-03-06 08:45:52 +0100 (Thu, 06 Mar 2008) | 2 lines Bug #2220: handle rlcompleter attribute match failure more gracefully. ........ r61278 | martin.v.loewis | 2008-03-06 14:49:47 +0100 (Thu, 06 Mar 2008) | 1 line Rely on x64 platform configuration when building _bsddb on AMD64. ........ r61279 | martin.v.loewis | 2008-03-06 14:50:28 +0100 (Thu, 06 Mar 2008) | 1 line Update db-4.4.20 build procedure. ........ r61285 | raymond.hettinger | 2008-03-06 21:52:01 +0100 (Thu, 06 Mar 2008) | 1 line More tests. ........ r61286 | raymond.hettinger | 2008-03-06 23:51:36 +0100 (Thu, 06 Mar 2008) | 1 line Issue 2246: itertools grouper object did not participate in GC (should be backported). ........ r61288 | raymond.hettinger | 2008-03-07 02:33:20 +0100 (Fri, 07 Mar 2008) | 1 line Tweak recipes and tests ........ r61289 | jeffrey.yasskin | 2008-03-07 07:22:15 +0100 (Fri, 07 Mar 2008) | 5 lines Progress on issue #1193577 by adding a polling .shutdown() method to SocketServers. The core of the patch was written by Pedro Werneck, but any bugs are mine. I've also rearranged the code for timeouts in order to avoid interfering with the shutdown poll. ........ r61290 | nick.coghlan | 2008-03-07 15:13:28 +0100 (Fri, 07 Mar 2008) | 1 line Speed up with statements by storing the __exit__ method on the stack instead of in a temp variable (bumps the magic number for pyc files) ........ r61298 | andrew.kuchling | 2008-03-07 22:09:23 +0100 (Fri, 07 Mar 2008) | 1 line Grammar fix ........ r61303 | georg.brandl | 2008-03-08 10:54:06 +0100 (Sat, 08 Mar 2008) | 2 lines #2253: fix continue vs. finally docs. ........ r61304 | marc-andre.lemburg | 2008-03-08 11:01:43 +0100 (Sat, 08 Mar 2008) | 3 lines Add new name for Mandrake: Mandriva. ........ r61305 | georg.brandl | 2008-03-08 11:05:24 +0100 (Sat, 08 Mar 2008) | 2 lines #1533486: fix types in refcount intro. ........ r61312 | facundo.batista | 2008-03-08 17:50:27 +0100 (Sat, 08 Mar 2008) | 5 lines Issue 1106316. post_mortem()'s parameter, traceback, is now optional: it defaults to the traceback of the exception that is currently being handled. ........ r61313 | jeffrey.yasskin | 2008-03-08 19:26:54 +0100 (Sat, 08 Mar 2008) | 2 lines Add tests for with and finally performance to pybench. ........ r61314 | jeffrey.yasskin | 2008-03-08 21:08:21 +0100 (Sat, 08 Mar 2008) | 2 lines Fix pybench for pythons < 2.6, tested back to 2.3. ........ r61317 | jeffrey.yasskin | 2008-03-08 22:35:15 +0100 (Sat, 08 Mar 2008) | 3 lines Well that was dumb. platform.python_implementation returns a function, not a string. ........ r61329 | georg.brandl | 2008-03-09 16:11:39 +0100 (Sun, 09 Mar 2008) | 2 lines #2249: document assertTrue and assertFalse. ........ r61332 | neal.norwitz | 2008-03-09 20:03:42 +0100 (Sun, 09 Mar 2008) | 4 lines Introduce a lock to fix a race condition which caused an exception in the test. Some buildbots were consistently failing (e.g., amd64). Also remove a couple of semi-colons. ........ r61344 | raymond.hettinger | 2008-03-11 01:19:07 +0100 (Tue, 11 Mar 2008) | 1 line Add recipe to docs. ........ r61350 | guido.van.rossum | 2008-03-11 22:18:06 +0100 (Tue, 11 Mar 2008) | 3 lines Fix the overflows in expandtabs(). "This time for sure!" (Exploit at request.) ........ r61351 | raymond.hettinger | 2008-03-11 22:37:46 +0100 (Tue, 11 Mar 2008) | 1 line Improve docs for itemgetter(). Show that it works with slices. ........ r61363 | georg.brandl | 2008-03-13 08:15:56 +0100 (Thu, 13 Mar 2008) | 2 lines #2265: fix example. ........ r61364 | georg.brandl | 2008-03-13 08:17:14 +0100 (Thu, 13 Mar 2008) | 2 lines #2270: fix typo. ........ r61365 | georg.brandl | 2008-03-13 08:21:41 +0100 (Thu, 13 Mar 2008) | 2 lines #1720705: add docs about import/threading interaction, wording by Nick. ........ r61366 | andrew.kuchling | 2008-03-13 12:07:35 +0100 (Thu, 13 Mar 2008) | 1 line Add class decorators ........ r61367 | raymond.hettinger | 2008-03-13 17:43:17 +0100 (Thu, 13 Mar 2008) | 1 line Add 2-to-3 support for the itertools moved to builtins or renamed. ........ r61368 | raymond.hettinger | 2008-03-13 17:43:59 +0100 (Thu, 13 Mar 2008) | 1 line Consistent tense. ........ r61369 | raymond.hettinger | 2008-03-13 20:03:51 +0100 (Thu, 13 Mar 2008) | 1 line Issue 2274: Add heapq.heappushpop(). ........ r61370 | raymond.hettinger | 2008-03-13 20:33:34 +0100 (Thu, 13 Mar 2008) | 1 line Simplify the nlargest() code using heappushpop(). ........ r61371 | brett.cannon | 2008-03-13 21:27:00 +0100 (Thu, 13 Mar 2008) | 4 lines Move test_thread over to unittest. Commits GHOP 237. Thanks Benjamin Peterson for the patch. ........ r61372 | brett.cannon | 2008-03-13 21:33:10 +0100 (Thu, 13 Mar 2008) | 4 lines Move test_tokenize to doctest. Done as GHOP 238 by Josip Dzolonga. ........ r61373 | brett.cannon | 2008-03-13 21:47:41 +0100 (Thu, 13 Mar 2008) | 4 lines Convert test_contains, test_crypt, and test_select to unittest. Patch from GHOP 294 by David Marek. ........ r61374 | brett.cannon | 2008-03-13 22:02:16 +0100 (Thu, 13 Mar 2008) | 4 lines Move test_gdbm to use unittest. Closes issue #1960. Thanks Giampaolo Rodola. ........ r61375 | brett.cannon | 2008-03-13 22:09:28 +0100 (Thu, 13 Mar 2008) | 4 lines Convert test_fcntl to unittest. Closes issue #2055. Thanks Giampaolo Rodola. ........ r61376 | raymond.hettinger | 2008-03-14 06:03:44 +0100 (Fri, 14 Mar 2008) | 1 line Leave heapreplace() unchanged. ........ r61378 | martin.v.loewis | 2008-03-14 14:56:09 +0100 (Fri, 14 Mar 2008) | 2 lines Patch #2284: add -x64 option to rt.bat. ........ r61379 | martin.v.loewis | 2008-03-14 14:57:59 +0100 (Fri, 14 Mar 2008) | 2 lines Use -x64 flag. ........ r61382 | brett.cannon | 2008-03-14 15:03:10 +0100 (Fri, 14 Mar 2008) | 2 lines Remove a bad test. ........ r61383 | mark.dickinson | 2008-03-14 15:23:37 +0100 (Fri, 14 Mar 2008) | 9 lines Issue 705836: Fix struct.pack(">f", 1e40) to behave consistently across platforms: it should now raise OverflowError on all platforms. (Previously it raised OverflowError only on non IEEE 754 platforms.) Also fix the (already existing) test for this behaviour so that it actually raises TestFailed instead of just referencing it. ........ r61387 | thomas.heller | 2008-03-14 22:06:21 +0100 (Fri, 14 Mar 2008) | 1 line Remove unneeded initializer. ........ r61388 | martin.v.loewis | 2008-03-14 22:19:28 +0100 (Fri, 14 Mar 2008) | 2 lines Run debug version, cd to PCbuild. ........ r61392 | georg.brandl | 2008-03-15 00:10:34 +0100 (Sat, 15 Mar 2008) | 2 lines Remove obsolete paragraph. #2288. ........ r61395 | georg.brandl | 2008-03-15 01:20:19 +0100 (Sat, 15 Mar 2008) | 2 lines Fix lots of broken links in the docs, found by Sphinx' external link checker. ........ r61396 | skip.montanaro | 2008-03-15 03:32:49 +0100 (Sat, 15 Mar 2008) | 1 line note that fork and forkpty raise OSError on failure ........ r61402 | skip.montanaro | 2008-03-15 17:04:45 +0100 (Sat, 15 Mar 2008) | 1 line add %f format to datetime - issue 1158 ........ r61403 | skip.montanaro | 2008-03-15 17:07:11 +0100 (Sat, 15 Mar 2008) | 2 lines . ........
2008-03-15 21:07:10 -03:00
.. _threaded-imports:
Importing in threaded code
--------------------------
While the import machinery is thread safe, there are two key
restrictions on threaded imports due to inherent limitations in the way
that thread safety is provided:
* Firstly, other than in the main module, an import should not have the
side effect of spawning a new thread and then waiting for that thread in
any way. Failing to abide by this restriction can lead to a deadlock if
the spawned thread directly or indirectly attempts to import a module.
* Secondly, all import attempts must be completed before the interpreter
starts shutting itself down. This can be most easily achieved by only
performing imports from non-daemon threads created through the threading
module. Daemon threads and threads created directly with the thread
module will require some other form of synchronization to ensure they do
not attempt imports after system shutdown has commenced. Failure to
abide by this restriction will lead to intermittent exceptions and
crashes during interpreter shutdown (as the late imports attempt to
access machinery which is no longer in a valid state).