Asynchronous context manager

asynchronous, contextmanager, python

Solution

Since Python 3.7, you can 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()

Before 3.7, 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()

Problem

I have an asynchronous API which I'm using to connect and send mail to an SMTP server which has some setup and tear down to it. So it fits nicely into using a `contextmanager` from Python 3's `contextlib`. Though, I don't know if it's possible write because they both use the generator syntax to write. This might demonstrate the problem (contains a mix of yield-base and async-await syntax to demonstrate the difference between async calls and yields to the context manager). ``` @contextmanager 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() ``` Is this kind of thing possible within python currently? and how would I use a `with` `as` statement if it is? If not is there a alternative way I could achieve this - maybe using the old style context manager?

Original source

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