Source code for traits_futures.background_progress

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# All rights reserved.
#
# This software is provided without warranty under the terms of the BSD
# license included in LICENSE.txt and may be redistributed only under
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"""
Support for a progress-reporting background call.

The code in this module supports an arbitrary callable that accepts a
"progress" named argument, and can use that argument to submit progress
information.

Every progress submission also marks a point where the callable can
be cancelled.
"""

from traits.api import Callable, Dict, Event, HasStrictTraits, Str, Tuple

from traits_futures.base_future import BaseFuture, BaseTask, TaskCancelled
from traits_futures.i_task_specification import ITaskSpecification

# Message types for messages from ProgressTask
# to ProgressFuture.

#: Task sends progress. Argument is a single object giving progress
#: information. This module does not interpret the contents of the argument.
PROGRESS = "progress"


class ProgressReporter:
    """
    Object used by the target callable to report progress.
    """

    def __init__(self, send, cancelled):
        self.send = send
        self.cancelled = cancelled

    def report(self, progress_info):
        """
        Send progress information to the linked future.

        The ``progress_info`` object will eventually be sent to the
        corresponding future's ``progress`` event trait.

        Parameters
        ----------
        progress_info : object
            An arbitrary object representing progress. Ideally, this
            should be immutable and pickleable.

        Raises
        ------
        TaskCancelled
            If a cancellation request for this task has already been made.
            In this case, the exception will be raised before any progress
            information is sent.
        """
        if self.cancelled():
            raise TaskCancelled("Task was cancelled")
        self.send(PROGRESS, progress_info)


class ProgressTask(BaseTask):
    """
    Background portion of a progress background task.

    This provides the callable that will be submitted to the worker pool, and
    sends messages to communicate with the ProgressFuture.
    """

    def __init__(self, callable, args, kwargs):
        self.callable = callable
        self.args = args
        self.kwargs = kwargs

    def run(self):
        progress = ProgressReporter(send=self.send, cancelled=self.cancelled)
        try:
            return self.callable(
                *self.args,
                **self.kwargs,
                progress=progress.report,
            )
        except TaskCancelled:
            return None


class ProgressFuture(BaseFuture):
    """
    Object representing the front-end handle to a ProgressTask.
    """

    #: Event fired whenever a progress message arrives from the background.
    progress = Event()

    # Private methods #########################################################

    def _process_progress(self, progress_info):
        self.progress = progress_info


@ITaskSpecification.register
class BackgroundProgress(HasStrictTraits):
    """
    Object representing the background task to be executed.
    """

    #: The callable to be executed.
    callable = Callable()

    #: Positional arguments to be passed to the callable.
    args = Tuple()

    #: Named arguments to be passed to the callable.
    kwargs = Dict(Str())

    def future(self, cancel):
        """
        Return a Future for the background task.

        Parameters
        ----------
        cancel
            Zero-argument callable, returning no useful result. The returned
            future's ``cancel`` method should call this to request cancellation
            of the associated background task.

        Returns
        -------
        future : ProgressFuture
            Future object that can be used to monitor the status of the
            background task.
        """
        return ProgressFuture(_cancel=cancel)

    def task(self):
        """
        Return a background callable for this task specification.

        Returns
        -------
        task : ProgressTask
            Callable accepting arguments ``send`` and ``cancelled``. The
            callable can use ``send`` to send messages and ``cancelled`` to
            check whether cancellation has been requested.
        """
        return ProgressTask(
            callable=self.callable,
            args=self.args,
            kwargs=self.kwargs,
        )


def submit_progress(executor, callable, *args, **kwargs):
    """
    Submit a progress-reporting task to an executor.

    Parameters
    ----------
    executor : TraitsExecutor
        Executor to submit the task to. This argument should always be passed
        by position rather than by name. Future versions of the library may
        enforce this restriction.
    callable
        Callable that executes the progress-providing function. This callable
        must accept a "progress" named argument, in addition to the provided
        arguments. The callable may then call the "progress" argument to report
        progress. This argument should always be passed by position rather than
        by name. Future versions of the library may enforce this restriction.
    *args
        Positional arguments to pass to the callable.
    **kwargs
        Named arguments other than "progress" to pass to the callable. These
        must not include "progress".

    Returns
    -------
    future : ProgressFuture
        Object representing the state of the background task.
    """
    if "progress" in kwargs:
        raise TypeError("progress may not be passed as a named argument")

    task = BackgroundProgress(callable=callable, args=args, kwargs=kwargs)
    return executor.submit(task)