481 lines
18 KiB
ReStructuredText
481 lines
18 KiB
ReStructuredText
:mod:`concurrent.futures` --- Launching parallel tasks
|
|
======================================================
|
|
|
|
.. module:: concurrent.futures
|
|
:synopsis: Execute computations concurrently using threads or processes.
|
|
|
|
.. versionadded:: 3.2
|
|
|
|
**Source code:** :source:`Lib/concurrent/futures/thread.py`
|
|
and :source:`Lib/concurrent/futures/process.py`
|
|
|
|
--------------
|
|
|
|
The :mod:`concurrent.futures` module provides a high-level interface for
|
|
asynchronously executing callables.
|
|
|
|
The asynchronous execution can be performed with threads, using
|
|
:class:`ThreadPoolExecutor`, or separate processes, using
|
|
:class:`ProcessPoolExecutor`. Both implement the same interface, which is
|
|
defined by the abstract :class:`Executor` class.
|
|
|
|
|
|
Executor Objects
|
|
----------------
|
|
|
|
.. class:: Executor
|
|
|
|
An abstract class that provides methods to execute calls asynchronously. It
|
|
should not be used directly, but through its concrete subclasses.
|
|
|
|
.. method:: submit(fn, *args, **kwargs)
|
|
|
|
Schedules the callable, *fn*, to be executed as ``fn(*args **kwargs)``
|
|
and returns a :class:`Future` object representing the execution of the
|
|
callable. ::
|
|
|
|
with ThreadPoolExecutor(max_workers=1) as executor:
|
|
future = executor.submit(pow, 323, 1235)
|
|
print(future.result())
|
|
|
|
.. method:: map(func, *iterables, timeout=None, chunksize=1)
|
|
|
|
Equivalent to :func:`map(func, *iterables) <map>` except *func* is executed
|
|
asynchronously and several calls to *func* may be made concurrently. The
|
|
returned iterator raises a :exc:`concurrent.futures.TimeoutError` if
|
|
:meth:`~iterator.__next__` is called and the result isn't available
|
|
after *timeout* seconds from the original call to :meth:`Executor.map`.
|
|
*timeout* can be an int or a float. If *timeout* is not specified or
|
|
``None``, there is no limit to the wait time. If a call raises an
|
|
exception, then that exception will be raised when its value is
|
|
retrieved from the iterator. When using :class:`ProcessPoolExecutor`, this
|
|
method chops *iterables* into a number of chunks which it submits to the
|
|
pool as separate tasks. The (approximate) size of these chunks can be
|
|
specified by setting *chunksize* to a positive integer. For very long
|
|
iterables, using a large value for *chunksize* can significantly improve
|
|
performance compared to the default size of 1. With :class:`ThreadPoolExecutor`,
|
|
*chunksize* has no effect.
|
|
|
|
.. versionchanged:: 3.5
|
|
Added the *chunksize* argument.
|
|
|
|
.. method:: shutdown(wait=True)
|
|
|
|
Signal the executor that it should free any resources that it is using
|
|
when the currently pending futures are done executing. Calls to
|
|
:meth:`Executor.submit` and :meth:`Executor.map` made after shutdown will
|
|
raise :exc:`RuntimeError`.
|
|
|
|
If *wait* is ``True`` then this method will not return until all the
|
|
pending futures are done executing and the resources associated with the
|
|
executor have been freed. If *wait* is ``False`` then this method will
|
|
return immediately and the resources associated with the executor will be
|
|
freed when all pending futures are done executing. Regardless of the
|
|
value of *wait*, the entire Python program will not exit until all
|
|
pending futures are done executing.
|
|
|
|
You can avoid having to call this method explicitly if you use the
|
|
:keyword:`with` statement, which will shutdown the :class:`Executor`
|
|
(waiting as if :meth:`Executor.shutdown` were called with *wait* set to
|
|
``True``)::
|
|
|
|
import shutil
|
|
with ThreadPoolExecutor(max_workers=4) as e:
|
|
e.submit(shutil.copy, 'src1.txt', 'dest1.txt')
|
|
e.submit(shutil.copy, 'src2.txt', 'dest2.txt')
|
|
e.submit(shutil.copy, 'src3.txt', 'dest3.txt')
|
|
e.submit(shutil.copy, 'src4.txt', 'dest4.txt')
|
|
|
|
|
|
ThreadPoolExecutor
|
|
------------------
|
|
|
|
:class:`ThreadPoolExecutor` is an :class:`Executor` subclass that uses a pool of
|
|
threads to execute calls asynchronously.
|
|
|
|
Deadlocks can occur when the callable associated with a :class:`Future` waits on
|
|
the results of another :class:`Future`. For example::
|
|
|
|
import time
|
|
def wait_on_b():
|
|
time.sleep(5)
|
|
print(b.result()) # b will never complete because it is waiting on a.
|
|
return 5
|
|
|
|
def wait_on_a():
|
|
time.sleep(5)
|
|
print(a.result()) # a will never complete because it is waiting on b.
|
|
return 6
|
|
|
|
|
|
executor = ThreadPoolExecutor(max_workers=2)
|
|
a = executor.submit(wait_on_b)
|
|
b = executor.submit(wait_on_a)
|
|
|
|
And::
|
|
|
|
def wait_on_future():
|
|
f = executor.submit(pow, 5, 2)
|
|
# This will never complete because there is only one worker thread and
|
|
# it is executing this function.
|
|
print(f.result())
|
|
|
|
executor = ThreadPoolExecutor(max_workers=1)
|
|
executor.submit(wait_on_future)
|
|
|
|
|
|
.. class:: ThreadPoolExecutor(max_workers=None, thread_name_prefix='', initializer=None, initargs=())
|
|
|
|
An :class:`Executor` subclass that uses a pool of at most *max_workers*
|
|
threads to execute calls asynchronously.
|
|
|
|
*initializer* is an optional callable that is called at the start of
|
|
each worker thread; *initargs* is a tuple of arguments passed to the
|
|
initializer. Should *initializer* raise an exception, all currently
|
|
pending jobs will raise a :exc:`~concurrent.futures.thread.BrokenThreadPool`,
|
|
as well any attempt to submit more jobs to the pool.
|
|
|
|
.. versionchanged:: 3.5
|
|
If *max_workers* is ``None`` or
|
|
not given, it will default to the number of processors on the machine,
|
|
multiplied by ``5``, assuming that :class:`ThreadPoolExecutor` is often
|
|
used to overlap I/O instead of CPU work and the number of workers
|
|
should be higher than the number of workers
|
|
for :class:`ProcessPoolExecutor`.
|
|
|
|
.. versionadded:: 3.6
|
|
The *thread_name_prefix* argument was added to allow users to
|
|
control the threading.Thread names for worker threads created by
|
|
the pool for easier debugging.
|
|
|
|
.. versionchanged:: 3.7
|
|
Added the *initializer* and *initargs* arguments.
|
|
|
|
|
|
.. _threadpoolexecutor-example:
|
|
|
|
ThreadPoolExecutor Example
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
::
|
|
|
|
import concurrent.futures
|
|
import urllib.request
|
|
|
|
URLS = ['http://www.foxnews.com/',
|
|
'http://www.cnn.com/',
|
|
'http://europe.wsj.com/',
|
|
'http://www.bbc.co.uk/',
|
|
'http://some-made-up-domain.com/']
|
|
|
|
# Retrieve a single page and report the URL and contents
|
|
def load_url(url, timeout):
|
|
with urllib.request.urlopen(url, timeout=timeout) as conn:
|
|
return conn.read()
|
|
|
|
# We can use a with statement to ensure threads are cleaned up promptly
|
|
with concurrent.futures.ThreadPoolExecutor(max_workers=5) as executor:
|
|
# Start the load operations and mark each future with its URL
|
|
future_to_url = {executor.submit(load_url, url, 60): url for url in URLS}
|
|
for future in concurrent.futures.as_completed(future_to_url):
|
|
url = future_to_url[future]
|
|
try:
|
|
data = future.result()
|
|
except Exception as exc:
|
|
print('%r generated an exception: %s' % (url, exc))
|
|
else:
|
|
print('%r page is %d bytes' % (url, len(data)))
|
|
|
|
|
|
ProcessPoolExecutor
|
|
-------------------
|
|
|
|
The :class:`ProcessPoolExecutor` class is an :class:`Executor` subclass that
|
|
uses a pool of processes to execute calls asynchronously.
|
|
:class:`ProcessPoolExecutor` uses the :mod:`multiprocessing` module, which
|
|
allows it to side-step the :term:`Global Interpreter Lock` but also means that
|
|
only picklable objects can be executed and returned.
|
|
|
|
The ``__main__`` module must be importable by worker subprocesses. This means
|
|
that :class:`ProcessPoolExecutor` will not work in the interactive interpreter.
|
|
|
|
Calling :class:`Executor` or :class:`Future` methods from a callable submitted
|
|
to a :class:`ProcessPoolExecutor` will result in deadlock.
|
|
|
|
.. class:: ProcessPoolExecutor(max_workers=None, mp_context=None, initializer=None, initargs=())
|
|
|
|
An :class:`Executor` subclass that executes calls asynchronously using a pool
|
|
of at most *max_workers* processes. If *max_workers* is ``None`` or not
|
|
given, it will default to the number of processors on the machine.
|
|
If *max_workers* is lower or equal to ``0``, then a :exc:`ValueError`
|
|
will be raised.
|
|
*mp_context* can be a multiprocessing context or None. It will be used to
|
|
launch the workers. If *mp_context* is ``None`` or not given, the default
|
|
multiprocessing context is used.
|
|
|
|
*initializer* is an optional callable that is called at the start of
|
|
each worker process; *initargs* is a tuple of arguments passed to the
|
|
initializer. Should *initializer* raise an exception, all currently
|
|
pending jobs will raise a :exc:`~concurrent.futures.thread.BrokenThreadPool`,
|
|
as well any attempt to submit more jobs to the pool.
|
|
|
|
.. versionchanged:: 3.3
|
|
When one of the worker processes terminates abruptly, a
|
|
:exc:`BrokenProcessPool` error is now raised. Previously, behaviour
|
|
was undefined but operations on the executor or its futures would often
|
|
freeze or deadlock.
|
|
|
|
.. versionchanged:: 3.7
|
|
The *mp_context* argument was added to allow users to control the
|
|
start_method for worker processes created by the pool.
|
|
|
|
Added the *initializer* and *initargs* arguments.
|
|
|
|
|
|
.. _processpoolexecutor-example:
|
|
|
|
ProcessPoolExecutor Example
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
::
|
|
|
|
import concurrent.futures
|
|
import math
|
|
|
|
PRIMES = [
|
|
112272535095293,
|
|
112582705942171,
|
|
112272535095293,
|
|
115280095190773,
|
|
115797848077099,
|
|
1099726899285419]
|
|
|
|
def is_prime(n):
|
|
if n % 2 == 0:
|
|
return False
|
|
|
|
sqrt_n = int(math.floor(math.sqrt(n)))
|
|
for i in range(3, sqrt_n + 1, 2):
|
|
if n % i == 0:
|
|
return False
|
|
return True
|
|
|
|
def main():
|
|
with concurrent.futures.ProcessPoolExecutor() as executor:
|
|
for number, prime in zip(PRIMES, executor.map(is_prime, PRIMES)):
|
|
print('%d is prime: %s' % (number, prime))
|
|
|
|
if __name__ == '__main__':
|
|
main()
|
|
|
|
|
|
Future Objects
|
|
--------------
|
|
|
|
The :class:`Future` class encapsulates the asynchronous execution of a callable.
|
|
:class:`Future` instances are created by :meth:`Executor.submit`.
|
|
|
|
.. class:: Future
|
|
|
|
Encapsulates the asynchronous execution of a callable. :class:`Future`
|
|
instances are created by :meth:`Executor.submit` and should not be created
|
|
directly except for testing.
|
|
|
|
.. method:: cancel()
|
|
|
|
Attempt to cancel the call. If the call is currently being executed and
|
|
cannot be cancelled then the method will return ``False``, otherwise the
|
|
call will be cancelled and the method will return ``True``.
|
|
|
|
.. method:: cancelled()
|
|
|
|
Return ``True`` if the call was successfully cancelled.
|
|
|
|
.. method:: running()
|
|
|
|
Return ``True`` if the call is currently being executed and cannot be
|
|
cancelled.
|
|
|
|
.. method:: done()
|
|
|
|
Return ``True`` if the call was successfully cancelled or finished
|
|
running.
|
|
|
|
.. method:: result(timeout=None)
|
|
|
|
Return the value returned by the call. If the call hasn't yet completed
|
|
then this method will wait up to *timeout* seconds. If the call hasn't
|
|
completed in *timeout* seconds, then a
|
|
:exc:`concurrent.futures.TimeoutError` will be raised. *timeout* can be
|
|
an int or float. If *timeout* is not specified or ``None``, there is no
|
|
limit to the wait time.
|
|
|
|
If the future is cancelled before completing then :exc:`.CancelledError`
|
|
will be raised.
|
|
|
|
If the call raised, this method will raise the same exception.
|
|
|
|
.. method:: exception(timeout=None)
|
|
|
|
Return the exception raised by the call. If the call hasn't yet
|
|
completed then this method will wait up to *timeout* seconds. If the
|
|
call hasn't completed in *timeout* seconds, then a
|
|
:exc:`concurrent.futures.TimeoutError` will be raised. *timeout* can be
|
|
an int or float. If *timeout* is not specified or ``None``, there is no
|
|
limit to the wait time.
|
|
|
|
If the future is cancelled before completing then :exc:`.CancelledError`
|
|
will be raised.
|
|
|
|
If the call completed without raising, ``None`` is returned.
|
|
|
|
.. method:: add_done_callback(fn)
|
|
|
|
Attaches the callable *fn* to the future. *fn* will be called, with the
|
|
future as its only argument, when the future is cancelled or finishes
|
|
running.
|
|
|
|
Added callables are called in the order that they were added and are
|
|
always called in a thread belonging to the process that added them. If
|
|
the callable raises an :exc:`Exception` subclass, it will be logged and
|
|
ignored. If the callable raises a :exc:`BaseException` subclass, the
|
|
behavior is undefined.
|
|
|
|
If the future has already completed or been cancelled, *fn* will be
|
|
called immediately.
|
|
|
|
The following :class:`Future` methods are meant for use in unit tests and
|
|
:class:`Executor` implementations.
|
|
|
|
.. method:: set_running_or_notify_cancel()
|
|
|
|
This method should only be called by :class:`Executor` implementations
|
|
before executing the work associated with the :class:`Future` and by unit
|
|
tests.
|
|
|
|
If the method returns ``False`` then the :class:`Future` was cancelled,
|
|
i.e. :meth:`Future.cancel` was called and returned `True`. Any threads
|
|
waiting on the :class:`Future` completing (i.e. through
|
|
:func:`as_completed` or :func:`wait`) will be woken up.
|
|
|
|
If the method returns ``True`` then the :class:`Future` was not cancelled
|
|
and has been put in the running state, i.e. calls to
|
|
:meth:`Future.running` will return `True`.
|
|
|
|
This method can only be called once and cannot be called after
|
|
:meth:`Future.set_result` or :meth:`Future.set_exception` have been
|
|
called.
|
|
|
|
.. method:: set_result(result)
|
|
|
|
Sets the result of the work associated with the :class:`Future` to
|
|
*result*.
|
|
|
|
This method should only be used by :class:`Executor` implementations and
|
|
unit tests.
|
|
|
|
.. method:: set_exception(exception)
|
|
|
|
Sets the result of the work associated with the :class:`Future` to the
|
|
:class:`Exception` *exception*.
|
|
|
|
This method should only be used by :class:`Executor` implementations and
|
|
unit tests.
|
|
|
|
|
|
Module Functions
|
|
----------------
|
|
|
|
.. function:: wait(fs, timeout=None, return_when=ALL_COMPLETED)
|
|
|
|
Wait for the :class:`Future` instances (possibly created by different
|
|
:class:`Executor` instances) given by *fs* to complete. Returns a named
|
|
2-tuple of sets. The first set, named ``done``, contains the futures that
|
|
completed (finished or were cancelled) before the wait completed. The second
|
|
set, named ``not_done``, contains uncompleted futures.
|
|
|
|
*timeout* can be used to control the maximum number of seconds to wait before
|
|
returning. *timeout* can be an int or float. If *timeout* is not specified
|
|
or ``None``, there is no limit to the wait time.
|
|
|
|
*return_when* indicates when this function should return. It must be one of
|
|
the following constants:
|
|
|
|
.. tabularcolumns:: |l|L|
|
|
|
|
+-----------------------------+----------------------------------------+
|
|
| Constant | Description |
|
|
+=============================+========================================+
|
|
| :const:`FIRST_COMPLETED` | The function will return when any |
|
|
| | future finishes or is cancelled. |
|
|
+-----------------------------+----------------------------------------+
|
|
| :const:`FIRST_EXCEPTION` | The function will return when any |
|
|
| | future finishes by raising an |
|
|
| | exception. If no future raises an |
|
|
| | exception then it is equivalent to |
|
|
| | :const:`ALL_COMPLETED`. |
|
|
+-----------------------------+----------------------------------------+
|
|
| :const:`ALL_COMPLETED` | The function will return when all |
|
|
| | futures finish or are cancelled. |
|
|
+-----------------------------+----------------------------------------+
|
|
|
|
.. function:: as_completed(fs, timeout=None)
|
|
|
|
Returns an iterator over the :class:`Future` instances (possibly created by
|
|
different :class:`Executor` instances) given by *fs* that yields futures as
|
|
they complete (finished or were cancelled). Any futures given by *fs* that
|
|
are duplicated will be returned once. Any futures that completed before
|
|
:func:`as_completed` is called will be yielded first. The returned iterator
|
|
raises a :exc:`concurrent.futures.TimeoutError` if :meth:`~iterator.__next__`
|
|
is called and the result isn't available after *timeout* seconds from the
|
|
original call to :func:`as_completed`. *timeout* can be an int or float. If
|
|
*timeout* is not specified or ``None``, there is no limit to the wait time.
|
|
|
|
|
|
.. seealso::
|
|
|
|
:pep:`3148` -- futures - execute computations asynchronously
|
|
The proposal which described this feature for inclusion in the Python
|
|
standard library.
|
|
|
|
|
|
Exception classes
|
|
-----------------
|
|
|
|
.. currentmodule:: concurrent.futures
|
|
|
|
.. exception:: CancelledError
|
|
|
|
Raised when a future is cancelled.
|
|
|
|
.. exception:: TimeoutError
|
|
|
|
Raised when a future operation exceeds the given timeout.
|
|
|
|
.. exception:: BrokenExecutor
|
|
|
|
Derived from :exc:`RuntimeError`, this exception class is raised
|
|
when an executor is broken for some reason, and cannot be used
|
|
to submit or execute new tasks.
|
|
|
|
.. versionadded:: 3.7
|
|
|
|
.. currentmodule:: concurrent.futures.thread
|
|
|
|
.. exception:: BrokenThreadPool
|
|
|
|
Derived from :exc:`~concurrent.futures.BrokenExecutor`, this exception
|
|
class is raised when one of the workers of a :class:`ThreadPoolExecutor`
|
|
has failed initializing.
|
|
|
|
.. versionadded:: 3.7
|
|
|
|
.. currentmodule:: concurrent.futures.process
|
|
|
|
.. exception:: BrokenProcessPool
|
|
|
|
Derived from :exc:`~concurrent.futures.BrokenExecutor` (formerly
|
|
:exc:`RuntimeError`), this exception class is raised when one of the
|
|
workers of a :class:`ProcessPoolExecutor` has terminated in a non-clean
|
|
fashion (for example, if it was killed from the outside).
|
|
|
|
.. versionadded:: 3.3
|