@mfeurer can you explain the restriction on the third part of the subcomponent?

Please see my answer in #773

@amueller shall I try taking this PR up?

@mfeurer about 4/5 unit tests throw the following error when running on the latest sklearn version locally
openml.exceptions.OpenMLServerException: Problem validating uploaded description file - XML does not correspond to XSD schema

Also, I couldn't understand why for task.class_labels which is annotated as Optional[List[str]], it gives a flake error if I index it task.class_labels[0].
Flake error: Value of type "Optional[List[str]]" is not indexable

Also, I couldn't understand why for task.class_labels which is annotated as Optional[List[str]], it gives a flake error if I index it task.class_labels[0].

As it is typed optional it can actually be None, which would not be indexable. You probably need to use typing.cast to please mypy.

openml.exceptions.OpenMLServerException: Problem validating uploaded description file - XML does not correspond to XSD schema

Hm, apparently, the dummy imputor needs a docstring, otherwise the description uploaded to OpenML will be emptly.

Codecov Report

Merging #777 into develop will decrease coverage by 0.42%.
The diff coverage is 68.83%.

@@             Coverage Diff             @@
##           develop     #777      +/-   ##
===========================================
- Coverage    88.57%   88.15%   -0.43%     
===========================================
  Files           37       37              
  Lines         4324     4363      +39     
===========================================
+ Hits          3830     3846      +16     
- Misses         494      517      +23

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Thanks for addressing my change requests, that all looks good now. There are a few old comments though, which are collapsed and you need to press 'show more to see them, that would be great if you could have a look at them. We can then ask @PGijsbers for another look and finally merge this :)