Async Context Manager

AsyncContextManager got introduced in conjunction with the async/await syntax in PEP 492 with Python 3.5.

AsyncContextManager

class AsyncContextManager:

    async def __aenter__(self):
        """Return context object upon entering the runtime context."""

    async def __aexit__(self, exc_type: Type[Exception], exc_value, traceback):
        """Raise any exception triggered within the runtime context."""

async with

Async Context Managers are used in conjunction with the async with statement.

The async with statement is defined as:

async with EXPRESSION as TARGET:
    BLOCK

where the as TARGET part is optional.

This is semantically equivalent to:

manager = (EXPRESSION)
try:
    TARGET = await manager.__aenter__()
    BLOCK
except:
    if not await manager.__aexit__(mgr, *sys.exc_info()):
        raise
else:
    await manager.__aexit__(mgr, None, None, None)

The best example for using an AsyncContextManager is creating a connection. While creating a protocol connection over a network the application code has to wait until the connection has been established. With an AsyncContextManager the code waiting for the I/O can suspend and some other task can run until the the connection is ready.

Summary

• AsyncContextManager can be used in conjunction with the async with statement.

• AsyncContextManager can suspend when entering the context, for example to establish a network connection.

• AsyncContextManagercan suspend while exiting, for example to close a network connection.