BUG/REG: multiindex nested tuples with duplicates

[jbrockmendel]

• closes value assignment bug for multi-indexed dataframes in v1.0.0rc0 #30892
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

there is an underlying problem with MultiIndex.get_loc that needs to get worked out, but that's going to take a while. This fixes the reported regression.

[TomAugspurger]

@jbrockmendel can you assign the 1.0.0 milestone for these regression fixes?

[jreback]

thanks @jbrockmendel

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

$ git checkout 1.0.x
$ git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 54a6490ed050a693f04c740af2dddbd2fd19c4e1

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am 'Backport PR #31358: BUG/REG: multiindex nested tuples with duplicates'

4. Push to a named branch :

git push YOURFORK 1.0.x:auto-backport-of-pr-31358-on-1.0.x

5. Create a PR against branch 1.0.x, I would have named this PR:

"Backport PR #31358 on branch 1.0.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to suggest an improvement.

[jreback]

@jbrockmendel can you push a backport for this, didn't work automatically.