2015-0492-Coroutines-with-async-and-await-syntax

PEP 492 – Coroutines with async and await syntax

Abstract

This PEP assumes that the asynchronous tasks are scheduled and coordinated by an Event Loop similar to that of stdlib module asyncio.events.AbstractEventLoop. While the PEP is not tied to any specific Event Loop implementation, it is relevant only to the kind of coroutine that uses yield as a signal to the scheduler, indicating that the coroutine will be waiting until an event (such as IO) is completed

Specification

New Coroutine Declaration Syntax

async def read_data(db):
    pass

Await Expression

async def read_data(db):
    data = await db.fetch('SELECT ...')
    ...

Asynchronous Context Managers and “async with”

class AsyncContextManager:
    async def __aenter__(self):
        await log('entering context')

    async def __aexit__(self, exc_type, exc, tb):
        await log('exiting context')

async with EXPR as VAR:
    BLOCK

Asynchronous Iterators and “async for”

class AsyncIterable:
    def __aiter__(self):
        return self

    async def __anext__(self):
        data = await self.fetch_data()
        if data:
            return data
        else:
            raise StopAsyncIteration

    async def fetch_data(self):
        ...

async for TARGET in ITER:
    BLOCK
else:
    BLOCK2

Glossary

• Native coroutine function
• Native coroutine
• Generator-based coroutine function
• Generator-based coroutine
• Coroutine
• Coroutine object
• Future-like object
• Awaitable
• Asynchronous context manager
• Asynchronous iterable
• Asynchronous iterator