diff options
Diffstat (limited to 'docs/index.rst')
| -rw-r--r-- | docs/index.rst | 347 |
1 files changed, 347 insertions, 0 deletions
diff --git a/docs/index.rst b/docs/index.rst new file mode 100644 index 0000000..d74cade --- /dev/null +++ b/docs/index.rst @@ -0,0 +1,347 @@ +:mod:`concurrent.futures` --- Asynchronous computation +====================================================== + +.. module:: concurrent.futures + :synopsis: Execute computations asynchronously using threads or processes. + +The :mod:`concurrent.futures` module provides a high-level interface for +asynchronously executing callables. + +The asynchronous execution can be be performed by 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` is an abstract class that provides methods to execute calls +asynchronously. It should not be used directly, but through its two +subclasses: :class:`ThreadPoolExecutor` and :class:`ProcessPoolExecutor`. + +.. method:: Executor.submit(fn, *args, **kwargs) + + Schedules the callable to be executed as *fn*(*\*args*, *\*\*kwargs*) and + returns a :class:`Future` representing the execution of the callable. + +:: + + with ThreadPoolExecutor(max_workers=1) as executor: + future = executor.submit(pow, 323, 1235) + print(future.result()) + +.. method:: Executor.map(func, *iterables, timeout=None) + + Equivalent to map(*func*, *\*iterables*) but func is executed asynchronously + and several calls to *func* may be made concurrently. The returned iterator + raises a :exc:`TimeoutError` if :meth:`__next__()` is called and the result + isn't available after *timeout* seconds from the original call to + :meth:`map()`. *timeout* can be an int or float. If *timeout* is not + specified or ``None`` then 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. + +.. method:: Executor.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 `with` + statement, which will shutdown the `Executor` (waiting as if + `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, 'src3.txt', 'dest4.txt') + + +ThreadPoolExecutor Objects +-------------------------- + +The :class:`ThreadPoolExecutor` class is an :class:`Executor` subclass that uses +a pool of threads to execute calls asynchronously. + +Deadlock 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) + + Executes calls asynchronously using a pool of at most *max_workers* threads. + +.. _threadpoolexecutor-example: + +ThreadPoolExecutor Example +^^^^^^^^^^^^^^^^^^^^^^^^^^ +:: + + from concurrent import 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/'] + + def load_url(url, timeout): + return urllib.request.urlopen(url, timeout=timeout).read() + + with futures.ThreadPoolExecutor(max_workers=5) as executor: + future_to_url = dict((executor.submit(load_url, url, 60), url) + for url in URLS) + + for future in futures.as_completed(future_to_url): + url = future_to_url[future] + if future.exception() is not None: + print('%r generated an exception: %s' % (url, + future.exception())) + else: + print('%r page is %d bytes' % (url, len(future.result()))) + +ProcessPoolExecutor Objects +--------------------------- + +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. + +Calling :class:`Executor` or :class:`Future` methods from a callable submitted +to a :class:`ProcessPoolExecutor` will result in deadlock. + +.. class:: ProcessPoolExecutor(max_workers=None) + + Executes calls asynchronously using a pool of at most *max_workers* + processes. If *max_workers* is ``None`` or not given then as many worker + processes will be created as the machine has processors. + +.. _processpoolexecutor-example: + +ProcessPoolExecutor Example +^^^^^^^^^^^^^^^^^^^^^^^^^^^ +:: + + 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 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 encapulates the asynchronous execution of a callable. +:class:`Future` instances are created by :meth:`Executor.submit`. + +.. method:: Future.cancel() + + Attempt to cancel the call. If the call is currently being executed then + it cannot be cancelled and the method will return `False`, otherwise the call + will be cancelled and the method will return `True`. + +.. method:: Future.cancelled() + + Return `True` if the call was successfully cancelled. + +.. method:: Future.running() + + Return `True` if the call is currently being executed and cannot be + cancelled. + +.. method:: Future.done() + + Return `True` if the call was successfully cancelled or finished running. + +.. method:: Future.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:`TimeoutError` will be raised. *timeout* can + be an int or float.If *timeout* is not specified or ``None`` then 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 then this method will raise the same exception. + +.. method:: Future.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:`TimeoutError` will be raised. + *timeout* can be an int or float. If *timeout* is not specified or ``None`` + then 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 then ``None`` is returned. + +.. method:: Future.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` then it will be logged and ignored. If the + callable raises another :exc:`BaseException` then the behavior is not + defined. + + If the future has already completed or been cancelled then *fn* will be + called immediately. + +Internal Future Methods +^^^^^^^^^^^^^^^^^^^^^^^ + +The following :class:`Future` methods are meant for use in unit tests and +:class:`Executor` implementations. + +.. method:: Future.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:: Future.set_result(result) + + Sets the result of the work associated with the :class:`Future` to *result*. + + This method should only be used by Executor implementations and unit tests. + +.. method:: Future.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 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`` then there is no limit to the wait time. + + *return_when* indicates when this function should return. It must be one of + the following constants: + + +-----------------------------+----------------------------------------+ + | 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 | + | | `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:`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. |