In CPython 3.5.2, the __aiter__ protocol was updated.

Before 3.5.2, __aiter__ was expected to return an awaitable resolving to an asynchronous iterator . Starting with 3.5.2, __aiter__ should return asynchronous iterators directly.

If the old protocol is used in 3.5.2, Python will raise a PendingDeprecationWarning .

In CPython 3.6, the old __aiter__ protocol will still be supported with a DeprecationWarning being raised.

In CPython 3.7, the old __aiter__ protocol will no longer be supported: a RuntimeError will be raised if __aiter__ returns anything but an asynchronous iterator.