The main change is dropping yield from support and using
async/await everywhere. Farewell, Python 3.4.
The minimal supported Python version is 3.5.3 now.
Why not 3.5.0? Because 3.5.3 has a crucial change: :func:`asyncio.get_event_loop()` returns the running loop instead of default, which may be different, e.g.:
loop = asyncio.new_event_loop() loop.run_until_complete(f())
Note, :func:`asyncio.set_event_loop` was not called and default loop is not equal to actually executed one.
People constantly asked about ability to run aiohttp servers together with other asyncio code, but :func:`aiohttp.web.run_app` is blocking synchronous call.
aiohttp had support for starting the application without run_app but the API
was very low-level and cumbersome.
Now application runners solve the task in a few lines of code, see :ref:`aiohttp-web-app-runners` for details.
Other long awaited feature is tracing client request life cycle to figure out when and why client request spends a time waiting for connection establishment, getting server response headers etc.
Now it is possible by registering special signal handlers on every request processing stage. :ref:`aiohttp-client-tracing` provides more info about the feature.
Unfortunately asyncio has a bug with checking SSL certificates for non-ASCII site DNS names, e.g. https://историк.рф or https://雜草工作室.香港.
The bug has been fixed in upcoming Python 3.7 only (the change requires breaking backward compatibility in :mod:`ssl` API).
aiohttp installs a fix for older Python versions (3.5 and 3.6).
A switch to new major version is a great chance for dropping already deprecated features.
The release dropped a lot, see :ref:`aiohttp_changes` for details.
All removals was already marked as deprecated or related to very low level implementation details.
If user code did not raise :exc:`DeprecationWarning` it is compatible with aiohttp 3.0 most likely.
Enjoy aiohttp 3.0 release!
The full change log is here: :ref:`aiohttp_changes`.