16.2. threading — Higher-level threading interface
This module constructs higher-level threading interfaces on top of the lower
level thread module.
See also the mutex and Queue modules.
The dummy_threading module is provided for situations where
threading cannot be used because thread is missing.
Note
Starting with Python 2.6, this module provides PEP 8 compliant aliases and
properties to replace the camelCase names that were inspired by Java’s
threading API. This updated API is compatible with that of the
multiprocessing module. However, no schedule has been set for the
deprecation of the camelCase names and they remain fully supported in
both Python 2.x and 3.x.
This module defines the following functions and objects:
-
threading.active_count()
-
threading.activeCount()
Return the number of Thread objects currently alive. The returned
count is equal to the length of the list returned by enumerate().
-
threading.Condition()
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.
-
threading.current_thread()
-
threading.currentThread()
Return the current Thread object, corresponding to the caller’s thread
of control. If the caller’s thread of control was not created through the
threading module, a dummy thread object with limited functionality is
returned.
-
threading.enumerate()
Return a list of all Thread objects currently alive. The list
includes daemonic threads, dummy thread objects created by
current_thread(), and the main thread. It excludes terminated threads
and threads that have not yet been started.
-
threading.Event()
A factory function that returns a new event object. An event manages a flag
that can be set to true with the set() method and reset to false
with the clear() method. The wait() method blocks until the flag
is true.
-
class
threading.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 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
_threading_local module.
New in version 2.4:
New in version 2.4.
-
threading.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.
-
threading.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.
-
threading.Semaphore([value])
A factory function that returns a new semaphore object. A semaphore manages a
counter representing the number of release() calls minus the number of
acquire() calls, plus an initial value. The acquire() method blocks
if necessary until it can return without making the counter negative. If not
given, value defaults to 1.
-
threading.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, 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
threading.Thread
A class that represents a thread of control. This class can be safely
subclassed in a limited fashion.
-
class
threading.Timer
A thread that executes a function after a specified interval has passed.
-
threading.settrace(func)
Set a trace function for all threads started from the threading module.
The func will be passed to sys.settrace() for each thread, before its
run() method is called.
New in version 2.3:
New in version 2.3.
-
threading.setprofile(func)
Set a profile function for all threads started from the threading module.
The func will be passed to sys.setprofile() for each thread, before its
run() method is called.
New in version 2.3:
New in version 2.3.
-
threading.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 ThreadError is raised. If the specified stack size is
invalid, a 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.
New in version 2.5:
New in version 2.5.
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 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.
16.2.1. 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 run() method in a subclass. No other
methods (except for the constructor) should be overridden in a subclass. In
other words, only override the __init__() and run() methods of
this class.
Once a thread object is created, its activity must be started by calling the
thread’s start() method. This invokes the run() method in a
separate thread of control.
Once the thread’s activity is started, the thread is considered ‘alive’. It
stops being alive when its run() method terminates – either normally, or
by raising an unhandled exception. The is_alive() method tests whether the
thread is alive.
Other threads can call a thread’s join() method. This blocks the calling
thread until the thread whose join() method is called is terminated.
A thread has a name. The name can be passed to the constructor, and read or
changed through the name attribute.
A thread can be flagged as a “daemon thread”. The significance of this flag is
that the entire Python program exits when only daemon threads are left. The
initial value is inherited from the creating thread. The flag can be set
through the daemon property.
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 join()ed. They are never deleted, since it is
impossible to detect the termination of alien threads.
-
class
threading.Thread(group=None, target=None, name=None, args=(), kwargs={})
This constructor should always be called with keyword arguments. Arguments
are:
group should be None; reserved for future extension when a
ThreadGroup class is implemented.
target is the callable object to be invoked by the 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.
-
start()
Start the thread’s activity.
It must be called at most once per thread object. It arranges for the
object’s run() method to be invoked in a separate thread of control.
This method will raise a RuntimeException if called more than once
on the same thread object.
-
run()
Method representing the thread’s activity.
You may override this method in a subclass. The standard run()
method invokes the callable object passed to the object’s constructor as
the target argument, if any, with sequential and keyword arguments taken
from the args and kwargs arguments, respectively.
-
join([timeout])
Wait until the thread terminates. This blocks the calling thread until the
thread whose join() method is called terminates – either normally
or through an unhandled exception – or until the optional timeout occurs.
When the timeout argument is present and not None, it should be a
floating point number specifying a timeout for the operation in seconds
(or fractions thereof). As join() always returns None, you must
call isAlive() after join() to decide whether a timeout
happened – if the thread is still alive, the 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 join()ed many times.
join() raises a RuntimeError if an attempt is made to join
the current thread as that would cause a deadlock. It is also an error to
join() a thread before it has been started and attempts to do so
raises the same exception.
-
getName()
-
setName()
Old API for name.
-
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.
-
ident
The ‘thread identifier’ of this thread or None if the thread has not
been started. This is a nonzero integer. See the
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.
New in version 2.6:
New in version 2.6.
-
is_alive()
-
isAlive()
Return whether the thread is alive.
This method returns True just before the run() method starts
until just after the run() method terminates. The module function
enumerate() returns a list of all alive threads.
-
isDaemon()
-
setDaemon()
Old API for daemon.
-
daemon
A boolean value indicating whether this thread is a daemon thread (True)
or not (False). This must be set before start() is called,
otherwise 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 daemon
= False.
The entire Python program exits when no alive non-daemon threads are left.
16.2.2. Lock Objects
A primitive lock is a synchronization primitive that is not owned by a
particular thread when locked. In Python, it is currently the lowest level
synchronization primitive available, implemented directly by the thread
extension module.
A primitive lock is in one of two states, “locked” or “unlocked”. It is created
in the unlocked state. It has two basic methods, acquire() and
release(). When the state is unlocked, acquire() changes the state
to locked and returns immediately. When the state is locked, acquire()
blocks until a call to release() in another thread changes it to unlocked,
then the acquire() call resets it to locked and returns. The
release() method should only be called in the locked state; it changes the
state to unlocked and returns immediately. If an attempt is made to release an
unlocked lock, a RuntimeError will be raised.
When more than one thread is blocked in acquire() waiting for the state to
turn to unlocked, only one thread proceeds when a release() call resets
the state to unlocked; which one of the waiting threads proceeds is not defined,
and may vary across implementations.
All methods are executed atomically.
-
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.
-
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.
16.2.3. 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 acquire() method; this returns once
the thread owns the lock. To unlock the lock, a thread calls its
release() method. acquire()/release() call pairs may be
nested; only the final release() (the release() of the outermost
pair) resets the lock to unlocked and allows another thread blocked in
acquire() to proceed.
-
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.
-
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
RuntimeError is raised if this method is called when the lock is
unlocked.
There is no return value.
16.2.4. 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 acquire() and release() methods that call
the corresponding methods of the associated lock. It also has a wait()
method, and notify() and notifyAll() methods. These three must only
be called when the calling thread has acquired the lock, otherwise a
RuntimeError is raised.
The wait() method releases the lock, and then blocks until it is awakened
by a notify() or notifyAll() call for the same condition variable in
another thread. Once awakened, it re-acquires the lock and returns. It is also
possible to specify a timeout.
The notify() method wakes up one of the threads waiting for the condition
variable, if any are waiting. The notifyAll() method wakes up all threads
waiting for the condition variable.
Note: the notify() and notifyAll() methods don’t release the lock;
this means that the thread or threads awakened will not return from their
wait() call immediately, but only when the thread that called
notify() or notifyAll() finally relinquishes ownership of the lock.
Tip: the typical programming style using condition variables uses the lock to
synchronize access to some shared state; threads that are interested in a
particular change of state call wait() repeatedly until they see the
desired state, while threads that modify the state call notify() or
notifyAll() 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
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()
To choose between notify() and notifyAll(), consider whether one
state change can be interesting for only one or several waiting threads. E.g.
in a typical producer-consumer situation, adding one item to the buffer only
needs to wake up one consumer thread.
-
class
threading.Condition([lock])
If the lock argument is given and not None, it must be a Lock
or RLock object, and it is used as the underlying lock. Otherwise,
a new RLock object is created and used as the underlying lock.
-
acquire(*args)
Acquire the underlying lock. This method calls the corresponding method on
the underlying lock; the return value is whatever that method returns.
-
release()
Release the underlying lock. This method calls the corresponding method on
the underlying lock; there is no return value.
-
wait([timeout])
Wait until notified or until a timeout occurs. If the calling thread has not
acquired the lock when this method is called, a RuntimeError is raised.
This method releases the underlying lock, and then blocks until it is
awakened by a notify() or notifyAll() call for the same
condition variable in another thread, or until the optional timeout
occurs. Once awakened or timed out, it re-acquires the lock and returns.
When the timeout argument is present and not None, it should be a
floating point number specifying a timeout for the operation in seconds
(or fractions thereof).
When the underlying lock is an RLock, it is not released using
its release() method, since this may not actually unlock the lock
when it was acquired multiple times recursively. Instead, an internal
interface of the 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.
-
notify()
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
RuntimeError is raised.
This method wakes up one of the threads waiting for the condition
variable, if any are waiting; it is a no-op if no threads are waiting.
The current implementation wakes up exactly one thread, if any are
waiting. However, it’s not safe to rely on this behavior. A future,
optimized implementation may occasionally wake up more than one thread.
Note: the awakened thread does not actually return from its wait()
call until it can reacquire the lock. Since notify() does not
release the lock, its caller should.
-
notify_all()
-
notifyAll()
Wake up all threads waiting on this condition. This method acts like
notify(), but wakes up all waiting threads instead of one. If the
calling thread has not acquired the lock when this method is called, a
RuntimeError is raised.
16.2.5. 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 P() and V() instead of acquire() and release()).
A semaphore manages an internal counter which is decremented by each
acquire() call and incremented by each release() call. The counter
can never go below zero; when acquire() finds that it is zero, it blocks,
waiting until some other thread calls release().
-
class
threading.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, ValueError is
raised.
-
acquire([blocking])
Acquire a semaphore.
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
release() to make it larger than zero. This is done with proper
interlocking so that if multiple acquire() calls are blocked,
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.
When invoked with blocking set to true, do the same thing as when called
without arguments, and return true.
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.
-
release()
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.
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.
16.2.6. Event Objects
This is one of the simplest mechanisms for communication between threads: one
thread signals an event and other threads wait for it.
An event object manages an internal flag that can be set to true with the
set() method and reset to false with the clear() method. The
wait() method blocks until the flag is true.
-
class
threading.Event
The internal flag is initially false.
-
is_set()
-
isSet()
Return true if and only if the internal flag is true.
Changed in version 2.6:
Changed in version 2.6: The is_set() syntax is new.
-
set()
Set the internal flag to true. All threads waiting for it to become true
are awakened. Threads that call wait() once the flag is true will
not block at all.
-
clear()
Reset the internal flag to false. Subsequently, threads calling
wait() will block until set() is called to set the internal
flag to true again.
-
wait([timeout])
Block until the internal flag is true. If the internal flag is true on
entry, return immediately. Otherwise, block until another thread calls
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 always returns None.
16.2.7. Timer Objects
This class represents an action that should be run only after a certain amount
of time has passed — a timer. Timer is a subclass of Thread
and as such also functions as an example of creating custom threads.
Timers are started, as with threads, by calling their start() method. The
timer can be stopped (before its action has begun) by calling the cancel()
method. The interval the timer will wait before executing its action may not be
exactly the same as the interval specified by the user.
For example:
def hello():
print "hello, world"
t = Timer(30.0, hello)
t.start() # after 30 seconds, "hello, world" will be printed
-
class
threading.Timer(interval, function, args=[], kwargs={})
Create a timer that will run function with arguments args and keyword
arguments kwargs, after interval seconds have passed.
-
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.
16.2.8. Using locks, conditions, and semaphores in the with statement
All of the objects provided by this module that have acquire() and
release() methods can be used as context managers for a with
statement. The acquire() method will be called when the block is entered,
and release() will be called when the block is exited.
Currently, Lock, RLock, Condition,
Semaphore, and BoundedSemaphore objects may be used as
with statement context managers. For example:
import threading
some_rlock = threading.RLock()
with some_rlock:
print "some_rlock is locked while this executes"
16.2.9. 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).