再次理解asyncio/await syntax and asyncio in Python

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• Overview

  asynchronous : 异步

  concurrent : 并发

  coroutine : 协程

  thread : 线程

  parallelism : 并行

  multiprocess : 多进程

• asyncio/await syntax

  PEP492 – Coroutines with async and await syntax

  The PEP492 is proposed to make coroutines a proper standalone concept in Python, and introduce new supporting syntax. The ultimate goal is to help establish a common, easily approachable, mental model of asynchronous programming in Python and make it as close to synchronous programming as possible.

  Current Python supports implementing coroutines via generators (PEP342), further enhanced by the yield from syntax introduced in PEP 380.

• New Coroutine Declaration Syntax

  the following new syntax is used to declare a native coroutine:

  async def read_data(db):pass
  

  Key properties of coroutines:

  • async def functions are always coroutines, even if they do not contain await expressions.
  • It is a SyntaxError to have yield or yield from expressions in an async function.

• Await Expression

  The following new await expression is used to obtain a result of coroutine execution :

  async def read_data(db):data = await db.fetch("select ...")...
  

  await, similarly to yield from, suspends execution of read_data coroutine until db.fetch awaitable completes and returns the result data.

  《理解awaitable in Python》

  It is a SyntaxError to use await outside of an async def function.

  It is a TypeError to pass anything other than an awaitable object to an await expression.

• Asynchronous Context Managers and “async with”

  An asynchronous context manager is a context manager that is able to suspend execution in its enter and exit methods.

  It is a SyntaxError to use async with outside of an async def function.

• asyncio - Asynchronous I/O

  廖雪峰的官方网站

  Async IO in Python: A Complete Walkthrough

  asyncio is a library to write concurrent code using the async/await syntax.

  asyncio 是Python3.4引入的标准库，async/await是Python3.5引入的针对asyncio的新语法。

  asyncio的编程模型是一个消息循环，从模型中直接获取一个EventLoop，然后把需要执行的携程扔到EventLoop中执行，实现异步。

  消息循环中有多个任务[a,b,c]：

  1. 首先执行第一个任务a,过程中遇到a内部的yield from或者await，a任务相当于continue，后续先不执行，等待await后面awaitful对象执行完成
  2. b任务执行，同样的遇到await之后continue b
  3. a任务中await任务完成，继续执行a过程后面的任务
  4. 如此继续

  不同coroutine是由同一个thread执行，即asyncio可以实现单线程并发IO操作。

• Low-level APIs

  Event loop is the core of every asyncio application. Event loops run asynchronous tasks and callbacks, perform network IO operations, and run subprocesses.

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