Asynchronous context manager

In Python 3.7, you'll be able to write:

from contextlib import asynccontextmanager

@asynccontextmanager
async def smtp_connection():
    client = SMTPAsync()
    ...

    try:
        await client.connect(smtp_url, smtp_port)
        await client.starttls()
        await client.login(smtp_username, smtp_password)
        yield client
    finally:
        await client.quit()

Until 3.7 comes out, you can use the async_generator package for this. On 3.6, you can write:

# This import changed, everything else is the same
from async_generator import asynccontextmanager

@asynccontextmanager
async def smtp_connection():
    client = SMTPAsync()
    ...

    try:
        await client.connect(smtp_url, smtp_port)
        await client.starttls()
        await client.login(smtp_username, smtp_password)
        yield client
    finally:
        await client.quit()

And if you want to work all the way back to 3.5, you can write:

# This import changed again:
from async_generator import asynccontextmanager, async_generator, yield_

@asynccontextmanager
@async_generator      # <-- added this
async def smtp_connection():
    client = SMTPAsync()
    ...

    try:
        await client.connect(smtp_url, smtp_port)
        await client.starttls()
        await client.login(smtp_username, smtp_password)
        await yield_(client)    # <-- this line changed
    finally:
        await client.quit()

Thanks to @jonrsharpe was able to make an async context manager.

Here's what mine ended up looking like for anyone who want's some example code:

class SMTPConnection():
    def __init__(self, url, port, username, password):
        self.client   = SMTPAsync()
        self.url      = url
        self.port     = port
        self.username = username
        self.password = password

    async def __aenter__(self):
        await self.client.connect(self.url, self.port)
        await self.client.starttls()
        await self.client.login(self.username, self.password)

        return self.client

    async def __aexit__(self, exc_type, exc, tb):
        await self.client.quit()

usage:

async with SMTPConnection(url, port, username, password) as client:
    await client.sendmail(...)

Feel free to point out if I've done anything stupid.

The asyncio_extras package has a nice solution for this:

import asyncio_extras

@asyncio_extras.async_contextmanager
async def smtp_connection():
    client = SMTPAsync()
    ...

For Python < 3.6, you'd also need the async_generator package and replace yield client with await yield_(client).