You have a rebase issue. I got once the problem. If I remember, I think that I have fixed
the issue using the following strategy

git checkout master
git fetch upstream
git rebase upstream/master
git checkout new-branch
git rebase master
git push -f 

Hope it helps.

@arjoly, appears to have worked. Thanks for the recommendation and sorry for the mistake.

Coverage decreased (-0.0%) when pulling 866e939 on mjbommar:label-encoder-unseen-final into 376ac51 on scikit-learn:master.

No problem, I ran once in that issue and this was frustrating. I am happy that it works for you!

Coverage decreased (-0.0%) when pulling da4cafb on mjbommar:label-encoder-unseen-final into 376ac51 on scikit-learn:master.

N.B.: Direct access to .classes_ should be replaced with .get_classes() in order to properly handle the new_labels="update".

But it's important to have a consistent API across the scikit. Could you use a property instead?

Coverage decreased (-0.01%) when pulling 0f3e3d3 on mjbommar:label-encoder-unseen-final into 376ac51 on scikit-learn:master.

@mblondel , thanks for the recommendation. Implemented as suggested.

@arjoly, @jnothman , anything I can do to make this easier for you to review?

@mjbommar, should we expect you won't be completing this any time soon and label it "needs contributor" for someone to adopt? I will do so, but you should say if you'd rather complete it.

@jnothman, my recollection is fuzzy, but I think this issue was primarily blocked by design disagreements. If we can come to an agreement about desired behavior, I could see how easily the work could be completed and merged into master.