Looking at this more closely, there are a bunch of things that can go wrong:

• cast(k) may fail,
• str(...) may fail,
• .cast<std::string>() may fail.

Any of those failures will be a secondary error, masking the desired KeyError. This can be extremely confusing.

Making this robust is not easy.

What's needed is something like (untested; only to give the idea):

str key_as_str;
try {
  key_as_str = str(cast(k));
}
catch (const std::exception&) {
    throw key_error();
}
set_error(PyExc_KeyError, key_as_tr);
throw error_already_set(); 

This would be implemented in a helper function.

Another thought: if str() fails, try repr().

Oh, there should also be a size limit. I.e. you don't want to produce a 1M numpy array as a str.

Sorry to make this difficult, but these things sure will bite, someone.

@rwgk PTAL. I added a test for long keys. I don't know what kind of object would fail in str of cast - happy to add a test if you have a suggestion. What is the benefit of set_error + throw error_already_set() over throw key_error() ?