reworked the _map method to make use of an arguments queue; added docstrings to the TaskPool methods

setup.cfg

@@ -1,6 +1,6 @@
 [metadata]
 name = asyncio-taskpool
-version = 0.0.3
+version = 0.1.1
 author = Daniil Fajnberg
 author_email = mail@daniil.fajnberg.de
 description = Dynamically manage pools of asyncio tasks

src/asyncio_taskpool/pool.py

@@ -3,6 +3,7 @@ from asyncio import gather
 from asyncio.coroutines import iscoroutine, iscoroutinefunction
 from asyncio.exceptions import CancelledError
 from asyncio.locks import Event, Semaphore
+from asyncio.queues import Queue, QueueEmpty
 from asyncio.tasks import Task, create_task
 from functools import partial
 from math import inf
@@ -39,8 +40,7 @@ class BaseTaskPool:
         self._num_ended: int = 0
         self._idx: int = self._add_pool(self)
         self._name: str = name
-        self._all_tasks_known_flag: Event = Event()
+        self._before_gathering: List[Awaitable] = []
-        self._all_tasks_known_flag.set()
         self._interrupt_flag: Event = Event()
         log.debug("%s initialized", str(self))
 
@@ -330,7 +330,7 @@ class BaseTaskPool:
         """
         if self._open:
             raise exceptions.PoolStillOpen("Pool must be closed, before tasks can be gathered")
-        await self._all_tasks_known_flag.wait()
+        await gather(*self._before_gathering)
         results = await gather(*self._ended.values(), *self._cancelled.values(), *self._running.values(),
                                return_exceptions=return_exceptions)
         self._ended = self._cancelled = self._running = {}
@@ -418,22 +418,72 @@ class TaskPool(BaseTaskPool):
         assert isinstance(ids, list)
         return ids
 
-    async def _next_callback(self, task_id: int, func: CoroutineFunc, args_iter: Iterator[Any], arg_stars: int = 0,
+    async def _queue_producer(self, q: Queue, args_iter: Iterator[Any]) -> None:
+        """
+        Keeps the arguments queue from `_map()` full as long as the iterator has elements.
+        If the `_interrupt_flag` gets set, the loop ends prematurely.
+
+        Args:
+            q:
+                The queue of function arguments to consume for starting the next task.
+            args_iter:
+                The iterator of function arguments to put into the queue.
+        """
+        for arg in args_iter:
+            if self._interrupt_flag.is_set():
+                break
+            await q.put(arg)  # This blocks as long as the queue is full.
+
+    async def _queue_consumer(self, q: Queue, func: CoroutineFunc, arg_stars: int = 0,
+                              end_callback: EndCallbackT = None, cancel_callback: CancelCallbackT = None) -> None:
+        """
+        Wrapper around the `_start_task()` taking the next element from the arguments queue set up in `_map()`.
+        Partially constructs the `_queue_callback` function with the same arguments.
+
+        Args:
+            q:
+                The queue of function arguments to consume for starting the next task.
+            func:
+                The coroutine function to use for spawning the tasks within the task pool.
+            arg_stars (optional):
+                Whether or not to unpack an element from `q` using stars; must be 0, 1, or 2.
+            end_callback (optional):
+                The actual callback specified to execute after the task (and the next one) has ended.
+                It is run with the task's ID as its only positional argument.
+            cancel_callback (optional):
+                The callback that was specified to execute after cancellation of the task (and the next one).
+                It is run with the task's ID as its only positional argument.
+        """
+        try:
+            arg = q.get_nowait()
+        except QueueEmpty:
+            return
+        try:
+            await self._start_task(
+                star_function(func, arg, arg_stars=arg_stars),
+                ignore_closed=True,
+                end_callback=partial(TaskPool._queue_callback, self, q=q, func=func, arg_stars=arg_stars,
+                                     end_callback=end_callback, cancel_callback=cancel_callback),
+                cancel_callback=cancel_callback
+            )
+        finally:
+            q.task_done()
+
+    async def _queue_callback(self, task_id: int, q: Queue, func: CoroutineFunc, arg_stars: int = 0,
                               end_callback: EndCallbackT = None, cancel_callback: CancelCallbackT = None) -> None:
         """
         Wrapper around an end callback function passed into the `_map()` method.
-        To be used in conjunction with `_start_next_task()` to simulate a queue of coroutines to be started as tasks
+        Triggers the next `_queue_consumer` with the same arguments.
-        in the pool, whenever the `_map()` method is called.
 
         Args:
             task_id:
                 The ID of the ending task.
+            q:
+                The queue of function arguments to consume for starting the next task.
             func:
                 The coroutine function to use for spawning the tasks within the task pool.
-            args_iter:
-                The iterator of arguments; each element is to be passed into a `func` call when spawning a new task.
             arg_stars (optional):
-                Whether or not to unpack an element from `args_iter` using stars; must be 0, 1, or 2.
+                Whether or not to unpack an element from `q` using stars; must be 0, 1, or 2.
             end_callback (optional):
                 The actual callback specified to execute after the task (and the next one) has ended.
                 It is run with the `task_id` as its only positional argument.
@@ -441,62 +491,19 @@ class TaskPool(BaseTaskPool):
                 The callback that was specified to execute after cancellation of the task (and the next one).
                 It is run with the `task_id` as its only positional argument.
         """
-        reached_end = await self._start_next_task(func, args_iter, arg_stars=arg_stars,
+        await self._queue_consumer(q, func, arg_stars, end_callback=end_callback, cancel_callback=cancel_callback)
-                                                  end_callback=end_callback, cancel_callback=cancel_callback)
-        if reached_end:
-            self._all_tasks_known_flag.set()
         await execute_optional(end_callback, args=(task_id,))
 
-    async def _start_next_task(self, func: CoroutineFunc, args_iter: Iterator[Any], arg_stars: int = 0,
-                               end_callback: EndCallbackT = None, cancel_callback: CancelCallbackT = None) -> bool:
-        """
-        Starts a new task in the pool using the next element from the arguments iterator.
-        Helper used in conjunction with the `_next_callback()` wrapper in the `_map()` method.
-
-        Args:
-            func:
-                The coroutine function to use for spawning the tasks within the task pool.
-            args_iter:
-                The iterator of arguments; each element is to be passed into a `func` call when spawning a new task.
-            arg_stars (optional):
-                Whether or not to unpack an element from `args_iter` using stars; must be 0, 1, or 2.
-            end_callback (optional):
-                The callback specified to execute after a task (and the next one) has ended.
-                It is run with the Task's ID as its only positional argument.
-            cancel_callback (optional):
-                The callback that was specified to execute after cancellation of the task (and the next one).
-                It is run with the Task's ID as its only positional argument.
-
-        Returns:
-            `True` if the end of `args_iter` has been reached or the `_interrupt_flag` has been set; `False` otherwise.
-        """
-        if self._interrupt_flag.is_set():
-            return True
-        try:
-            await self._start_task(
-                star_function(func, next(args_iter), arg_stars=arg_stars),
-                ignore_closed=True,
-                end_callback=partial(TaskPool._next_callback, self, func=func, args_iter=args_iter, arg_stars=arg_stars,
-                                     end_callback=end_callback, cancel_callback=cancel_callback),
-                cancel_callback=cancel_callback
-            )
-        except StopIteration:
-            return True
-        return False
-
     async def _map(self, func: CoroutineFunc, args_iter: ArgsT, arg_stars: int = 0, num_tasks: int = 1,
                    end_callback: EndCallbackT = None, cancel_callback: CancelCallbackT = None) -> None:
         """
-        Creates coroutines with arguments from a supplied iterable and runs them as new tasks in the pool in chunks.
+        Creates coroutines with arguments from a supplied iterable and runs them as new tasks in the pool in batches.
         Each coroutine looks like `func(arg)`, `func(*arg)`, or `func(**arg)`, `arg` being an element from the iterable.
 
-        This method blocks, **only if** there is not enough room in the pool for the first chunk of new tasks.
+        This method blocks, **only if** there is not enough room in the pool for the first batch of new tasks.
 
-        It clears the internal `_all_tasks_known_flag` until the end of the iterable of arguments has been reached,
+        It sets up an internal queue which is filled while consuming the arguments iterable.
-        then sets it.
+        The queue's `join()` method is added to the pool's `_before_gathering` list.
-        TODO: This is wrong because it may interfere with another call to this method.
-              Consider rebuilding this entire method using a `asyncio.Queue` instead of convoluted callbacks.
-              Then instead of the `_all_tasks_known_flag` the pool's `gather` can wait on the Queue's `join`...
 
         Args:
             func:
@@ -519,27 +526,84 @@ class TaskPool(BaseTaskPool):
         """
         if not self.is_open:
             raise exceptions.PoolIsClosed("Cannot start new tasks")
-        if self._all_tasks_known_flag.is_set():
+        args_queue = Queue(maxsize=num_tasks)
-            self._all_tasks_known_flag.clear()
+        self._before_gathering.append(args_queue.join())
         args_iter = iter(args_iter)
+        try:
+            # Here we guarantee that the queue will contain as many arguments as needed for starting the first batch of
+            # tasks, which will be at most `num_tasks` (meaning the queue will be full).
+            for i in range(num_tasks):
+                args_queue.put_nowait(next(args_iter))
+        except StopIteration:
+            # If we get here, this means that the number of elements in the arguments iterator was less than the
+            # specified `num_tasks`. Thus, the number of tasks to start immediately will be the size of the queue.
+            # The `_queue_producer` won't be necessary, since we already put all the elements in the queue.
+            num_tasks = args_queue.qsize()
+        else:
+            # There may be more elements in the arguments iterator, so we need the `_queue_producer`.
+            # It will have exclusive access to the `args_iter` from now on.
+            # If the queue is full already, it will wait until one of the tasks in the first batch ends, before putting
+            # the next item in it.
+            create_task(self._queue_producer(args_queue, args_iter))
         for _ in range(num_tasks):
-            reached_end = await self._start_next_task(func, args_iter, arg_stars, end_callback, cancel_callback)
+            # This is where blocking can occur, if the pool is full.
-            if reached_end:
+            await self._queue_consumer(args_queue, func,
-                self._all_tasks_known_flag.set()
+                                       arg_stars=arg_stars, end_callback=end_callback, cancel_callback=cancel_callback)
-                break
 
-    async def map(self, func: CoroutineFunc, args_iter: ArgsT, num_tasks: int = 1,
+    async def map(self, func: CoroutineFunc, arg_iter: ArgsT, num_tasks: int = 1,
                   end_callback: EndCallbackT = None, cancel_callback: CancelCallbackT = None) -> None:
-        await self._map(func, args_iter, arg_stars=0, num_tasks=num_tasks,
+        """
+        An asyncio-task-based equivalent of the `multiprocessing.pool.Pool.map` method.
+
+        Creates coroutines with arguments from a supplied iterable and runs them as new tasks in the pool in batches.
+        Each coroutine looks like `func(arg)`, `arg` being an element from the iterable.
+
+        Once the first batch of tasks has started to run, this method returns.
+        As soon as on of them finishes, it triggers the start of a new task (assuming there is room in the pool)
+        consuming the next element from the arguments iterable.
+        If the size of the pool never imposes a limit, this ensures that there is almost continuously the desired number
+        of tasks from this call concurrently running within the pool.
+
+        This method blocks, **only if** there is not enough room in the pool for the first batch of new tasks.
+
+        Args:
+            func:
+                The coroutine function to use for spawning the new tasks within the task pool.
+            arg_iter:
+                The iterable of arguments; each argument is to be passed into a `func` call when spawning a new task.
+            num_tasks (optional):
+                The maximum number of the new tasks to run concurrently.
+            end_callback (optional):
+                A callback to execute after a task has ended.
+                It is run with the task's ID as its only positional argument.
+            cancel_callback (optional):
+                A callback to execute after cancellation of a task.
+                It is run with the task's ID as its only positional argument.
+
+        Raises:
+            `PoolIsClosed` if the pool has been closed.
+            `NotCoroutine` if `func` is not a coroutine function.
+        """
+        await self._map(func, arg_iter, arg_stars=0, num_tasks=num_tasks,
                         end_callback=end_callback, cancel_callback=cancel_callback)
 
     async def starmap(self, func: CoroutineFunc, args_iter: Iterable[ArgsT], num_tasks: int = 1,
                       end_callback: EndCallbackT = None, cancel_callback: CancelCallbackT = None) -> None:
+        """
+        Like `map()` except that the elements of `args_iter` are expected to be iterables themselves to be unpacked as
+        positional arguments to the function.
+        Each coroutine then looks like `func(*arg)`, `arg` being an element from `args_iter`.
+        """
         await self._map(func, args_iter, arg_stars=1, num_tasks=num_tasks,
                         end_callback=end_callback, cancel_callback=cancel_callback)
 
     async def doublestarmap(self, func: CoroutineFunc, kwargs_iter: Iterable[KwArgsT], num_tasks: int = 1,
                             end_callback: EndCallbackT = None, cancel_callback: CancelCallbackT = None) -> None:
+        """
+        Like `map()` except that the elements of `kwargs_iter` are expected to be iterables themselves to be unpacked as
+        keyword-arguments to the function.
+        Each coroutine then looks like `func(**arg)`, `arg` being an element from `kwargs_iter`.
+        """
         await self._map(func, kwargs_iter, arg_stars=2, num_tasks=num_tasks,
                         end_callback=end_callback, cancel_callback=cancel_callback)