How about to add more friendly exception if plain func is going to be registered as handler? I guess now such code will fail with cryptic TypeError: object NoneType can't be used in 'await' expression or similar.
Well, but I see you point.
Maybe we need specialized CheckedFrozenList class with registered callback? It could improve UX slightly.
Please let me merge the PR as is and make a new issue.
I don't want to block @pfreixes with his client tracing PR.
This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new
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