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Commit 035cedb4 authored by Georg Brandl's avatar Georg Brandl
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Edit concurrent docs, add versionadded and see also reference to the PEP.

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Doc/library/concurrent.futures.rst
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.. module:: concurrent.futures
:synopsis: Execute computations concurrently using threads or processes.
.. versionadded:: 3.2
The :mod:`concurrent.futures` module provides a high-level interface for
asynchronously executing callables.
The asynchronous execution can be be performed with threads, using
:class:`ThreadPoolExecutor`, or seperate processes, using
:class:`ProcessPoolExecutor`. Both implement the same interface, which is
: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
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.
callable. ::
::
with ThreadPoolExecutor(max_workers=1) as executor:
future = executor.submit(pow, 323, 1235)
print(future.result())
with ThreadPoolExecutor(max_workers=1) as executor:
future = executor.submit(pow, 323, 1235)
print(future.result())
.. method:: map(func, *iterables, timeout=None)
Equivalent to ``map(func, *iterables)`` except *func* is executed
asynchronously and several calls to *func* may be made concurrently. The
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:`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.
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.
.. 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
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
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
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`):
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, 'src3.txt', 'dest4.txt')
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
^^^^^^^^^^^^^^^^^^
------------------
:class:`ThreadPoolExecutor` is a :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
the results of another :class:`Future`. For example::
def wait_on_a():
time.sleep(5)
print(a.result()) # a will never complete because it is waiting on b.
return 6
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:
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())
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)
executor = ThreadPoolExecutor(max_workers=1)
executor.submit(wait_on_future)
.. class:: ThreadPoolExecutor(max_workers)
......@@ -117,39 +114,40 @@ And:
An :class:`Executor` subclass that uses a pool of at most *max_workers*
threads to execute calls asynchronously.
.. _threadpoolexecutor-example:
ThreadPoolExecutor Example
^^^^^^^^^^^^^^^^^^^^^^^^^^
~~~~~~~~~~~~~~~~~~~~~~~~~~
::
import concurrent.futures
import urllib.request
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/']
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()
def load_url(url, timeout):
return urllib.request.urlopen(url, timeout=timeout).read()
with concurrent.futures.ThreadPoolExecutor(max_workers=5) as executor:
future_to_url = dict((executor.submit(load_url, url, 60), url)
for url in URLS)
with concurrent.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 concurrent.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())))
for future in concurrent.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
^^^^^^^^^^^^^^^^^^^
-------------------
The :class:`ProcessPoolExecutor` class is an :class:`Executor` subclass that
uses a pool of processes to execute calls asynchronously.
......@@ -163,59 +161,61 @@ 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
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.
.. _processpoolexecutor-example:
ProcessPoolExecutor Example
^^^^^^^^^^^^^^^^^^^^^^^^^^^
~~~~~~~~~~~~~~~~~~~~~~~~~~~
::
import concurrent.futures
import math
import concurrent.futures
import math
PRIMES = [
112272535095293,
112582705942171,
112272535095293,
115280095190773,
115797848077099,
1099726899285419]
PRIMES = [
112272535095293,
112582705942171,
112272535095293,
115280095190773,
115797848077099,
1099726899285419]
def is_prime(n):
if n % 2 == 0:
return False
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
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))
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()
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`.
.. class:: Future
Encapulates the asynchronous execution of a callable. :class:`Future`
Encapulates 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
Attempt to cancel the call. If the call is currently being executed and
cannot be cancelled and the method will return ``False``, otherwise the
call will be cancelled and the method will return ``True``.
......@@ -236,9 +236,9 @@ The :class:`Future` class encapulates the asynchronous execution of a callable.
.. 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
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
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`
......@@ -248,11 +248,11 @@ The :class:`Future` class encapulates the asynchronous execution of a callable.
.. 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:`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.
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``, there is no limit to the wait time.
If the future is cancelled before completing then :exc:`CancelledError`
will be raised.
......@@ -261,14 +261,14 @@ The :class:`Future` class encapulates the asynchronous execution of a callable.
.. method:: add_done_callback(fn)
Attaches the callable *fn* to the future. *fn* will be called, with the
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
always called in a thread belonging to the process that added them. If
the callable raises a :exc:`Exception` subclass, it will be logged and
ignored. If the callable raises a :exc:`BaseException` subclass, the
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
......@@ -280,16 +280,16 @@ The :class:`Future` class encapulates the asynchronous execution of a callable.
.. 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.
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 `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
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
......@@ -314,46 +314,53 @@ The :class:`Future` class encapulates the asynchronous execution of a callable.
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
: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.
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
*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 |
| | :const:`ALL_COMPLETED`. |
+-----------------------------+----------------------------------------+
| :const:`ALL_COMPLETED` | The function will return when all |
| | futures finish or are cancelled. |
+-----------------------------+----------------------------------------+
+-----------------------------+----------------------------------------+
| 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 that completed before
:func:`as_completed` is called will be yielded first. 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
:func:`as_completed`. *timeout* can be an int or float. If *timeout* is not
they complete (finished or were cancelled). Any futures that completed
before :func:`as_completed` is called will be yielded first. 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
: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.
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