.. module:: evergreen.futures Futures ======= This module implements an (almost) API compatible `concurrent.futures` implementation which is cooperative. .. py:class:: Future The Future class encapsulates the asynchronous execution of a callable. Future instances are created by Executor.submit(). .. py: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. .. py:attribute:: cancelled Return True if the call was successfully cancelled. .. py:attribute:: done Return True if the call was successfully cancelled or finished running. .. py:method:: 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. .. py:method:: 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. .. py:class:: Executor An abstract class that provides methods to execute calls asynchronously. It should not be used directly, but through its concrete subclasses. .. py: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()) .. py: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 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 :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. .. py: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 `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') .. py:class:: TaskPoolExecutor(max_workers) An :class:`Executor` subclass that uses a pool of at most `max_workers` tasks to execute calls concurrently. .. py:class:: ThreadPoolExecutor(max_workers) An :class:`Executor` subclass that uses a pool of at most `max_workers` threads to execute calls asynchronously. .. py: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: +-----------------------------+----------------------------------------+ | 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. | +-----------------------------+----------------------------------------+ .. py:function:: as_completed 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:`~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. Exceptions ---------- .. py:exception:: CancelledError .. py:exception:: 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