Futures

This module implements an (almost) API compatible concurrent.futures implementation which is cooperative.

class evergreen.futures.Future

The Future class encapsulates the asynchronous execution of a callable. Future instances are created by Executor.submit().

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.

cancelled

Return True if the call was successfully cancelled.

done

Return True if the call was successfully cancelled or finished running.

get(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 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 CancelledError will be raised.

If the call raised, this method will raise the same exception.

add_done_callback(func)

Attaches the callable func to the future. func 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 a Exception subclass, it will be logged and ignored. If the callable raises a BaseException subclass, the behavior is undefined.

If the future has already completed or been cancelled, func will be called immediately.

class evergreen.futures.Executor

An abstract class that provides methods to execute calls asynchronously. It should not be used directly, but through its concrete subclasses.

submit(fn, *args, **kwargs)

Schedules the callable, fn, to be executed as fn(*args **kwargs) and returns a Future object representing the execution of the callable.

with ThreadPoolExecutor(max_workers=1) as executor:
    future = executor.submit(pow, 323, 1235)
    print(future.result())

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 returned iterator raises a TimeoutError if __next__() is called and the result isn’t available after timeout seconds from the original call to 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.

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 Executor.submit() and Executor.map() made after shutdown will raise 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')

class evergreen.futures.TaskPoolExecutor(max_workers)

An Executor subclass that uses a pool of at most max_workers tasks to execute calls concurrently.

class evergreen.futures.ThreadPoolExecutor(max_workers)

An Executor subclass that uses a pool of at most max_workers threads to execute calls asynchronously.

evergreen.futures.wait(fs, timeout=None, return_when=ALL_COMPLETED)

Wait for the Future instances (possibly created by different 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:

Constant	Description
FIRST_COMPLETED	The function will return when any future finishes or is cancelled.
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.
ALL_COMPLETED	The function will return when all futures finish or are cancelled.

evergreen.futures.as_completed()

Returns an iterator over the Future instances (possibly created by different Executor instances) given by fs that yields futures as they complete (finished or were cancelled). Any futures that completed before as_completed() is called will be yielded first. The returned iterator raises a TimeoutError if __next__() is called and the result isn’t available after timeout seconds from the original call to as_completed(). timeout can be an int or float. If timeout is not specified or None, there is no limit to the wait time.

Exceptions

exception evergreen.futures.CancelledError

exception evergreen.futures.TimeoutError

Future class API changes

The future class in this module doesn’t conform 100% to the API exposed by the equivalent class in the concurrent.futures module from the standard library, though they are pretty minor. Here is the list of changes:

• cancelled and done are properties, not functions
• result function is called get
• there is no exception function
• there is no running function
• futures can only be used once, after the result (or exception) is fetched from a future, it will raise RuntimeError if get() is called again on it