See similar issue for SimpleNamespace: https://bugs.python.org/issue31655.

We should add a similar check in str.format(). Accepting a non-string keys is an implementation detail. There is no guarantee that it works in other implementations.

It should not hit performance, because the kwargs dict is usually small, the check is cheap, and formatting itself can be expensive. For the case when you need to pass a huge dict and use only few items from it, you can use str.format_map().