| :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) |
| |
| An :class:`Executor` subclass that uses a pool of at most *max_workers* |
| threads to execute calls asynchronously. |
| |
| .. 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`. |
| |
| |
| .. _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) |
| |
| 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. |
| |
| .. 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. |
| |
| |
| .. _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. |
| |
| .. currentmodule:: concurrent.futures.process |
| |
| .. exception:: BrokenProcessPool |
| |
| Derived from :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 |
| |