A small addition that can be added here is
The optional methods have to be initialized as an empty function example:
def instantiate_model_from_hpo_class():
"This is just an empty function to avoid errors"

It can be confusing for some users to get errors that this optional method is not implemented. So here we can add that though these methods are optional they should still be present in the extension as an empty function.

What do you mean exactly? I would just suggest users to raise a NotImplementedError (since that method will only be invoked externally)

In the end, you still need these optional methods to exist in the extension, While using the extension openml-python does require the optional methods to exist too(at least that was the issue I faced while developing openml-tensorflow). So I just defined the functions without definitions like here: https://github.com/openml/openml-tensorflow/blob/master/openml_tensorflow/extension.py

But it was confusing for me at first when I was getting those errors, so I think it will be helpful to add a message regarding that here(that we still need to define these methods as empty methods if we don't want to implement them).

In the end, you still need these optional methods to exist in the extension

Are you sure? Since the extensin inherits from the Extension interface, we technically do not need to implement it in the specific right?

Just to be sure, I added:

Note that although this method is optional, for the extension to work it
needs to exist. If this function is not required, we recommend to have an
empty function that raises a NotImplementedError with a useful error
message.

The docbuild fails because this indentation will lead to a warning (which we don't allow). In case you have issues building docs locally there's also a docker image available.

should I remove all line breaks?

Line breaks are OK, but it's indented too far for this parser. Content for the lists should only be indented as far as the asterisks (not sure why the parser wants this, to be honest).

I think this is a misleading error message. I added a line break and it now works. I created a commit which fixes doc building and will now create a commit fixing the merge conflicts arising from this.

Shouldn't it rather be that the implementing the logic is optional but that the function needs to exists? So

Note that although this method is required, it is not necessary to implement any logic if hyperparameter optimization is not implemented. Simply raise a `NotImplementedError` then.