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--- wiki2:python3 [2020/06/20 12:50]
alfred [The event loop]
+++ wiki2:python3 [2022/10/12 19:03] (actual)
@@ Línea 88: / Línea 88: @@
 Co-routines are functions that in a way similar to green threads are executed in a single thread and process. However, they are not so costly as are not managed by the operating system but by the **event loop**.
+For an advance use go to [[wiki2:python:notes#advanced_asyncio|Advanced asyncio]].
 ==== Basic points ====
@@ Línea 260: / Línea 261: @@
 ''create_task'' and ''create_future'' . There is also the ''set_task_factory'' if you want to create your own task instances.
-=== Futures and executors ===
+==== Futures and executors ====
+A ''Future'' represents an eventual result of an asynchronous operation.
+<code python>
+async def set_after(fut, delay, value):
+    await asyncio.sleep(delay)
+    fut.set_result(value)
+async def main():
+    loop = asyncio.get_running_loop()
+    fut = loop.create_future()
+    loop.create_task(set_after(fut, 1, '... world'))  # We could have just used "asyncio.create_task()".
+    print('hello ...')
+    print(await fut)  # Wait until *fut* has a result (1 second) and print it.
+asyncio.run(main())
+</code>
+An instance of ''Executor'' provides methods to execute calls asynchronously. It should not be used directly, but through its concrete subclasses.
+<code python>
+async def main():
+    loop = asyncio.get_running_loop()
+    result = await loop.run_in_executor(      # 1. Returns a Future.Executor
+        None, blocking_io)
+    print('default thread pool', result)
+    with concurrent.futures.ThreadPoolExecutor() as pool:      # 2. Run in a custom thread pool:
+        result = await loop.run_in_executor(
+            pool, blocking_io)
+        print('custom thread pool', result)
+    with concurrent.futures.ProcessPoolExecutor() as pool:    # 3. Run in a custom process pool:
+        result = await loop.run_in_executor(
+            pool, cpu_bound)
+        print('custom process pool', result)
+</code>
+''ThreadPoolExecutor'' uses a pool of threads in the same way than ''ProcessPoolExecutor'' uses a pool of processes.
+<code python>
+executor = ThreadPoolExecutor(max_workers=2)
+a = executor.submit(wait_on_b)
+b = executor.submit(wait_on_a)
+</code>
+==== Other tools ====
+=== Streams ===
+  * [[https://docs.python.org/3/library/asyncio-stream.html]]
+These allow to manage IO resources (sockets) asynchronously.
+=== Synchronization ===
+  * [[https://docs.python.org/3/library/asyncio-sync.html]]
+You can make use of the usual tools for synchronize co-routines (Lock, Event, Condition, Semaphore, BoundedSemaphore).
+=== Queues ===
+  * [[https://docs.python.org/3/library/asyncio-queue.html]]
+Asynchronous queues that can be used to distribute workload between several concurrent tasks:
+<code python>
+import asyncio
+import random
+import time
+async def worker(name, queue):
+    while True:
+        sleep_for = await queue.get()
+        await asyncio.sleep(sleep_for)
+        queue.task_done()
+async def main():
+    queue = asyncio.Queue()
+    total_sleep_time = 0
+    for _ in range(20):
+        sleep_for = random.uniform(0.05, 1.0)
+        total_sleep_time += sleep_for
+        queue.put_nowait(sleep_for)
+    # Create three worker tasks to process the queue concurrently.
+    tasks = []
+    for i in range(3):
+        task = asyncio.create_task(worker(f'worker-{i}', queue))
+        tasks.append(task)
+    # Wait until the queue is fully processed.
+    started_at = time.monotonic()
+    await queue.join()
+    total_slept_for = time.monotonic() - started_at
+    # Cancel our worker tasks.
+    for task in tasks:
+        task.cancel()
+    # Wait until all worker tasks are cancelled.
+    await asyncio.gather(*tasks, return_exceptions=True)
+    print('====')
+    print(f'3 workers slept in parallel for {total_slept_for:.2f} seconds')
+    print(f'total expected sleep time: {total_sleep_time:.2f} seconds')
+asyncio.run(main())
+</code>
 ==== Notes on co-routines ====
@@ Línea 298: / Línea 393: @@
   * [[https://news.ycombinator.com/item?id=23498742|Discussion on Hacker News]]
+=== functools.partial for passing arguments ===
+''functools.partial'' allows to create a function that not requires arguments from one that requires them.
+<code python>
+loop.call_soon(partial(print, "Hello", flush=True))
+</code>
+=== Stop co-routines when signals ===
+<code python>
+import asyncio
+import functools
+import os
+import signal
+def ask_exit(signame, loop):
+    print("got signal %s: exit" % signame)
+    loop.stop()
+async def main():
+    loop = asyncio.get_running_loop()
+    for signame in {'SIGINT', 'SIGTERM'}:
+        loop.add_signal_handler(
+            getattr(signal, signame),
+            functools.partial(ask_exit, signame, loop))
+    await asyncio.sleep(3600)
+print("Event loop running for 1 hour, press Ctrl+C to interrupt.")
+print(f"pid {os.getpid()}: send SIGINT or SIGTERM to exit.")
+asyncio.run(main())
+</code>
+===== Enums =====
+<code python>
+from enum import Enum
+class Numbers(Enum):
+    ONE = 1
+    TWO = 2
+number = Numbers(2)  # <Numbers.TWO: 2>
+number1 = Numbers["ONE"]  # <Numbers.TWO: 2>
+members = [n for n in Numbers]  # [<Numbers.ONE: 1>, <Numbers.TWO: 2>]
+values = [n.value for n in Numbers]  # [1, 2]
+</code>
+==== Links ====
+  * https://realpython.com/python-enum/
+===== Python type checking =====
+<code>
+pi: float = 3.142
+def headline(text: str, align: bool = True) -> str:
+    pass
+</code>
+The ''MyPy'' library allows to check code files.
+<code>
+>>> names: list = ["Guido", "Jukka", "Ivan"]
+>>> version: tuple = (3, 7, 1)
+>>> options: dict = {"centered": False, "capitalize": True}
+>>> from typing import Dict, List, Tuple
+>>> names: List[str] = ["Guido", "Jukka", "Ivan"]
+>>> version: Tuple[int, int, int] = (3, 7, 1)
+>>> options: Dict[str, bool] = {"centered": False, "capitalize": True}
+</code>
+<code>
+import random
+from typing import Any, Sequence
+def choose(items: Sequence[Any]) -> Any:
+    return random.choice(items)
+</code>
+<code>
+class Animal:
+    def __init__(self, name: str, birthday: date) -> None:
+        self.name = name
+        self.birthday = birthday
+    @classmethod
+    def newborn(cls, name: str) -> "Animal":
+        return cls(name, date.today())
+    def twin(self, name: str) -> "Animal":
+        cls = self.__class__
+        return cls(name, self.birthday)
+</code>
+<code>
+def headline(text, width=80, fill_char="-"):
+    # type: (str, int, str) -> str
+    return f" {text.title()} ".center(width, fill_char)
+def headline(
+    text,           # type: str
+    width=80,       # type: int
+    fill_char="-",  # type: str
+):                  # type: (...) -> str
+    return f" {text.title()} ".center(width, fill_char)
+</code>
 ===== Notes =====

Programming

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