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ReStructuredText
537 lines
20 KiB
ReStructuredText
:mod:`concurrent.futures` --- Launching parallel tasks
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======================================================
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.. module:: concurrent.futures
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:synopsis: Execute computations concurrently using threads or processes.
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.. versionadded:: 3.2
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**Source code:** :source:`Lib/concurrent/futures/thread.py`
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and :source:`Lib/concurrent/futures/process.py`
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--------------
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The :mod:`concurrent.futures` module provides a high-level interface for
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asynchronously executing callables.
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The asynchronous execution can be performed with threads, using
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:class:`ThreadPoolExecutor`, or separate processes, using
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:class:`ProcessPoolExecutor`. Both implement the same interface, which is
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defined by the abstract :class:`Executor` class.
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Executor Objects
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----------------
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.. class:: Executor
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An abstract class that provides methods to execute calls asynchronously. It
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should not be used directly, but through its concrete subclasses.
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.. method:: submit(fn, /, *args, **kwargs)
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Schedules the callable, *fn*, to be executed as ``fn(*args **kwargs)``
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and returns a :class:`Future` object representing the execution of the
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callable. ::
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with ThreadPoolExecutor(max_workers=1) as executor:
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future = executor.submit(pow, 323, 1235)
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print(future.result())
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.. method:: map(func, *iterables, timeout=None, chunksize=1)
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Similar to :func:`map(func, *iterables) <map>` except:
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* the *iterables* are collected immediately rather than lazily;
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* *func* is executed asynchronously and several calls to
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*func* may be made concurrently.
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The returned iterator raises a :exc:`concurrent.futures.TimeoutError`
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if :meth:`~iterator.__next__` is called and the result isn't available
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after *timeout* seconds from the original call to :meth:`Executor.map`.
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*timeout* can be an int or a float. If *timeout* is not specified or
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``None``, there is no limit to the wait time.
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If a *func* call raises an exception, then that exception will be
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raised when its value is retrieved from the iterator.
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When using :class:`ProcessPoolExecutor`, this method chops *iterables*
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into a number of chunks which it submits to the pool as separate
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tasks. The (approximate) size of these chunks can be specified by
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setting *chunksize* to a positive integer. For very long iterables,
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using a large value for *chunksize* can significantly improve
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performance compared to the default size of 1. With
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:class:`ThreadPoolExecutor`, *chunksize* has no effect.
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.. versionchanged:: 3.5
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Added the *chunksize* argument.
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.. method:: shutdown(wait=True, *, cancel_futures=False)
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Signal the executor that it should free any resources that it is using
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when the currently pending futures are done executing. Calls to
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:meth:`Executor.submit` and :meth:`Executor.map` made after shutdown will
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raise :exc:`RuntimeError`.
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If *wait* is ``True`` then this method will not return until all the
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pending futures are done executing and the resources associated with the
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executor have been freed. If *wait* is ``False`` then this method will
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return immediately and the resources associated with the executor will be
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freed when all pending futures are done executing. Regardless of the
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value of *wait*, the entire Python program will not exit until all
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pending futures are done executing.
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If *cancel_futures* is ``True``, this method will cancel all pending
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futures that the executor has not started running. Any futures that
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are completed or running won't be cancelled, regardless of the value
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of *cancel_futures*.
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If both *cancel_futures* and *wait* are ``True``, all futures that the
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executor has started running will be completed prior to this method
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returning. The remaining futures are cancelled.
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You can avoid having to call this method explicitly if you use the
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:keyword:`with` statement, which will shutdown the :class:`Executor`
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(waiting as if :meth:`Executor.shutdown` were called with *wait* set to
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``True``)::
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import shutil
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with ThreadPoolExecutor(max_workers=4) as e:
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e.submit(shutil.copy, 'src1.txt', 'dest1.txt')
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e.submit(shutil.copy, 'src2.txt', 'dest2.txt')
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e.submit(shutil.copy, 'src3.txt', 'dest3.txt')
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e.submit(shutil.copy, 'src4.txt', 'dest4.txt')
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.. versionchanged:: 3.9
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Added *cancel_futures*.
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ThreadPoolExecutor
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------------------
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:class:`ThreadPoolExecutor` is an :class:`Executor` subclass that uses a pool of
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threads to execute calls asynchronously.
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Deadlocks can occur when the callable associated with a :class:`Future` waits on
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the results of another :class:`Future`. For example::
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import time
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def wait_on_b():
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time.sleep(5)
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print(b.result()) # b will never complete because it is waiting on a.
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return 5
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def wait_on_a():
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time.sleep(5)
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print(a.result()) # a will never complete because it is waiting on b.
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return 6
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executor = ThreadPoolExecutor(max_workers=2)
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a = executor.submit(wait_on_b)
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b = executor.submit(wait_on_a)
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And::
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def wait_on_future():
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f = executor.submit(pow, 5, 2)
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# This will never complete because there is only one worker thread and
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# it is executing this function.
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print(f.result())
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executor = ThreadPoolExecutor(max_workers=1)
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executor.submit(wait_on_future)
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.. class:: ThreadPoolExecutor(max_workers=None, thread_name_prefix='', initializer=None, initargs=())
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An :class:`Executor` subclass that uses a pool of at most *max_workers*
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threads to execute calls asynchronously.
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*initializer* is an optional callable that is called at the start of
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each worker thread; *initargs* is a tuple of arguments passed to the
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initializer. Should *initializer* raise an exception, all currently
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pending jobs will raise a :exc:`~concurrent.futures.thread.BrokenThreadPool`,
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as well as any attempt to submit more jobs to the pool.
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.. versionchanged:: 3.5
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If *max_workers* is ``None`` or
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not given, it will default to the number of processors on the machine,
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multiplied by ``5``, assuming that :class:`ThreadPoolExecutor` is often
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used to overlap I/O instead of CPU work and the number of workers
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should be higher than the number of workers
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for :class:`ProcessPoolExecutor`.
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.. versionadded:: 3.6
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The *thread_name_prefix* argument was added to allow users to
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control the :class:`threading.Thread` names for worker threads created by
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the pool for easier debugging.
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.. versionchanged:: 3.7
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Added the *initializer* and *initargs* arguments.
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.. versionchanged:: 3.8
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Default value of *max_workers* is changed to ``min(32, os.cpu_count() + 4)``.
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This default value preserves at least 5 workers for I/O bound tasks.
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It utilizes at most 32 CPU cores for CPU bound tasks which release the GIL.
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And it avoids using very large resources implicitly on many-core machines.
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ThreadPoolExecutor now reuses idle worker threads before starting
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*max_workers* worker threads too.
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.. _threadpoolexecutor-example:
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ThreadPoolExecutor Example
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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::
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import concurrent.futures
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import urllib.request
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URLS = ['http://www.foxnews.com/',
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'http://www.cnn.com/',
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'http://europe.wsj.com/',
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'http://www.bbc.co.uk/',
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'http://some-made-up-domain.com/']
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# Retrieve a single page and report the URL and contents
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def load_url(url, timeout):
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with urllib.request.urlopen(url, timeout=timeout) as conn:
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return conn.read()
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# We can use a with statement to ensure threads are cleaned up promptly
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with concurrent.futures.ThreadPoolExecutor(max_workers=5) as executor:
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# Start the load operations and mark each future with its URL
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future_to_url = {executor.submit(load_url, url, 60): url for url in URLS}
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for future in concurrent.futures.as_completed(future_to_url):
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url = future_to_url[future]
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try:
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data = future.result()
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except Exception as exc:
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print('%r generated an exception: %s' % (url, exc))
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else:
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print('%r page is %d bytes' % (url, len(data)))
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ProcessPoolExecutor
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-------------------
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The :class:`ProcessPoolExecutor` class is an :class:`Executor` subclass that
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uses a pool of processes to execute calls asynchronously.
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:class:`ProcessPoolExecutor` uses the :mod:`multiprocessing` module, which
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allows it to side-step the :term:`Global Interpreter Lock
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<global interpreter lock>` but also means that
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only picklable objects can be executed and returned.
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The ``__main__`` module must be importable by worker subprocesses. This means
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that :class:`ProcessPoolExecutor` will not work in the interactive interpreter.
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Calling :class:`Executor` or :class:`Future` methods from a callable submitted
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to a :class:`ProcessPoolExecutor` will result in deadlock.
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.. class:: ProcessPoolExecutor(max_workers=None, mp_context=None, initializer=None, initargs=())
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An :class:`Executor` subclass that executes calls asynchronously using a pool
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of at most *max_workers* processes. If *max_workers* is ``None`` or not
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given, it will default to the number of processors on the machine.
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If *max_workers* is less than or equal to ``0``, then a :exc:`ValueError`
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will be raised.
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On Windows, *max_workers* must be less than or equal to ``61``. If it is not
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then :exc:`ValueError` will be raised. If *max_workers* is ``None``, then
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the default chosen will be at most ``61``, even if more processors are
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available.
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*mp_context* can be a multiprocessing context or None. It will be used to
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launch the workers. If *mp_context* is ``None`` or not given, the default
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multiprocessing context is used.
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*initializer* is an optional callable that is called at the start of
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each worker process; *initargs* is a tuple of arguments passed to the
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initializer. Should *initializer* raise an exception, all currently
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pending jobs will raise a :exc:`~concurrent.futures.process.BrokenProcessPool`,
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as well as any attempt to submit more jobs to the pool.
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.. versionchanged:: 3.3
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When one of the worker processes terminates abruptly, a
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:exc:`BrokenProcessPool` error is now raised. Previously, behaviour
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was undefined but operations on the executor or its futures would often
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freeze or deadlock.
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.. versionchanged:: 3.7
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The *mp_context* argument was added to allow users to control the
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start_method for worker processes created by the pool.
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Added the *initializer* and *initargs* arguments.
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.. _processpoolexecutor-example:
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ProcessPoolExecutor Example
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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::
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import concurrent.futures
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import math
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PRIMES = [
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112272535095293,
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112582705942171,
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112272535095293,
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115280095190773,
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115797848077099,
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1099726899285419]
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def is_prime(n):
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if n < 2:
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return False
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if n == 2:
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return True
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if n % 2 == 0:
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return False
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sqrt_n = int(math.floor(math.sqrt(n)))
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for i in range(3, sqrt_n + 1, 2):
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if n % i == 0:
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return False
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return True
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def main():
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with concurrent.futures.ProcessPoolExecutor() as executor:
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for number, prime in zip(PRIMES, executor.map(is_prime, PRIMES)):
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print('%d is prime: %s' % (number, prime))
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if __name__ == '__main__':
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main()
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Future Objects
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--------------
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The :class:`Future` class encapsulates the asynchronous execution of a callable.
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:class:`Future` instances are created by :meth:`Executor.submit`.
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.. class:: Future
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Encapsulates the asynchronous execution of a callable. :class:`Future`
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instances are created by :meth:`Executor.submit` and should not be created
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directly except for testing.
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.. method:: cancel()
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Attempt to cancel the call. If the call is currently being executed or
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finished running and cannot be cancelled then the method will return
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``False``, otherwise the call will be cancelled and the method will
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return ``True``.
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.. method:: cancelled()
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Return ``True`` if the call was successfully cancelled.
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.. method:: running()
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Return ``True`` if the call is currently being executed and cannot be
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cancelled.
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.. method:: done()
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Return ``True`` if the call was successfully cancelled or finished
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running.
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.. method:: result(timeout=None)
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Return the value returned by the call. If the call hasn't yet completed
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then this method will wait up to *timeout* seconds. If the call hasn't
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completed in *timeout* seconds, then a
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:exc:`concurrent.futures.TimeoutError` will be raised. *timeout* can be
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an int or float. If *timeout* is not specified or ``None``, there is no
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limit to the wait time.
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If the future is cancelled before completing then :exc:`.CancelledError`
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will be raised.
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If the call raised, this method will raise the same exception.
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.. method:: exception(timeout=None)
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Return the exception raised by the call. If the call hasn't yet
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completed then this method will wait up to *timeout* seconds. If the
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call hasn't completed in *timeout* seconds, then a
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:exc:`concurrent.futures.TimeoutError` will be raised. *timeout* can be
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an int or float. If *timeout* is not specified or ``None``, there is no
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limit to the wait time.
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If the future is cancelled before completing then :exc:`.CancelledError`
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will be raised.
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If the call completed without raising, ``None`` is returned.
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.. method:: add_done_callback(fn)
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Attaches the callable *fn* to the future. *fn* will be called, with the
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future as its only argument, when the future is cancelled or finishes
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running.
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Added callables are called in the order that they were added and are
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always called in a thread belonging to the process that added them. If
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the callable raises an :exc:`Exception` subclass, it will be logged and
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ignored. If the callable raises a :exc:`BaseException` subclass, the
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behavior is undefined.
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If the future has already completed or been cancelled, *fn* will be
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called immediately.
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The following :class:`Future` methods are meant for use in unit tests and
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:class:`Executor` implementations.
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.. method:: set_running_or_notify_cancel()
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This method should only be called by :class:`Executor` implementations
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before executing the work associated with the :class:`Future` and by unit
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tests.
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If the method returns ``False`` then the :class:`Future` was cancelled,
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i.e. :meth:`Future.cancel` was called and returned `True`. Any threads
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waiting on the :class:`Future` completing (i.e. through
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:func:`as_completed` or :func:`wait`) will be woken up.
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If the method returns ``True`` then the :class:`Future` was not cancelled
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and has been put in the running state, i.e. calls to
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:meth:`Future.running` will return `True`.
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This method can only be called once and cannot be called after
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:meth:`Future.set_result` or :meth:`Future.set_exception` have been
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called.
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.. method:: set_result(result)
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Sets the result of the work associated with the :class:`Future` to
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*result*.
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This method should only be used by :class:`Executor` implementations and
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unit tests.
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.. versionchanged:: 3.8
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This method raises
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:exc:`concurrent.futures.InvalidStateError` if the :class:`Future` is
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already done.
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.. method:: set_exception(exception)
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Sets the result of the work associated with the :class:`Future` to the
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:class:`Exception` *exception*.
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This method should only be used by :class:`Executor` implementations and
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unit tests.
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.. versionchanged:: 3.8
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This method raises
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:exc:`concurrent.futures.InvalidStateError` if the :class:`Future` is
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already done.
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Module Functions
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----------------
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.. function:: wait(fs, timeout=None, return_when=ALL_COMPLETED)
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Wait for the :class:`Future` instances (possibly created by different
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:class:`Executor` instances) given by *fs* to complete. Returns a named
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2-tuple of sets. The first set, named ``done``, contains the futures that
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completed (finished or cancelled futures) before the wait completed. The
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second set, named ``not_done``, contains the futures that did not complete
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(pending or running futures).
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*timeout* can be used to control the maximum number of seconds to wait before
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returning. *timeout* can be an int or float. If *timeout* is not specified
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or ``None``, there is no limit to the wait time.
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*return_when* indicates when this function should return. It must be one of
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the following constants:
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.. tabularcolumns:: |l|L|
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+-----------------------------+----------------------------------------+
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| Constant | Description |
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+=============================+========================================+
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| :const:`FIRST_COMPLETED` | The function will return when any |
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| | future finishes or is cancelled. |
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+-----------------------------+----------------------------------------+
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| :const:`FIRST_EXCEPTION` | The function will return when any |
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| | future finishes by raising an |
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| | exception. If no future raises an |
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| | exception then it is equivalent to |
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| | :const:`ALL_COMPLETED`. |
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+-----------------------------+----------------------------------------+
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| :const:`ALL_COMPLETED` | The function will return when all |
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| | futures finish or are cancelled. |
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+-----------------------------+----------------------------------------+
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.. function:: as_completed(fs, timeout=None)
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Returns an iterator over the :class:`Future` instances (possibly created by
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different :class:`Executor` instances) given by *fs* that yields futures as
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they complete (finished or cancelled futures). Any futures given by *fs* that
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are duplicated will be returned once. Any futures that completed before
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:func:`as_completed` is called will be yielded first. The returned iterator
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raises a :exc:`concurrent.futures.TimeoutError` if :meth:`~iterator.__next__`
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is called and the result isn't available after *timeout* seconds from the
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original call to :func:`as_completed`. *timeout* can be an int or float. If
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*timeout* is not specified or ``None``, there is no limit to the wait time.
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.. seealso::
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:pep:`3148` -- futures - execute computations asynchronously
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The proposal which described this feature for inclusion in the Python
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standard library.
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Exception classes
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-----------------
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.. currentmodule:: concurrent.futures
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.. exception:: CancelledError
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Raised when a future is cancelled.
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.. exception:: TimeoutError
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Raised when a future operation exceeds the given timeout.
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.. exception:: BrokenExecutor
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Derived from :exc:`RuntimeError`, this exception class is raised
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when an executor is broken for some reason, and cannot be used
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to submit or execute new tasks.
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.. versionadded:: 3.7
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.. exception:: InvalidStateError
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Raised when an operation is performed on a future that is not allowed
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in the current state.
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.. versionadded:: 3.8
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.. currentmodule:: concurrent.futures.thread
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.. exception:: BrokenThreadPool
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Derived from :exc:`~concurrent.futures.BrokenExecutor`, this exception
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class is raised when one of the workers of a :class:`ThreadPoolExecutor`
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has failed initializing.
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.. versionadded:: 3.7
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.. currentmodule:: concurrent.futures.process
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.. exception:: BrokenProcessPool
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Derived from :exc:`~concurrent.futures.BrokenExecutor` (formerly
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:exc:`RuntimeError`), this exception class is raised when one of the
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workers of a :class:`ProcessPoolExecutor` has terminated in a non-clean
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fashion (for example, if it was killed from the outside).
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.. versionadded:: 3.3
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