BUG: .loc with MultiIndex with tuple in level GH#27591

[jbrockmendel]

• closes BUG: MultiIndex Slice returns incorrect slice (compared to .xs()) #27591
• tests added / passed
• Ensure all linting tests pass, see here for how to run them
• whatsnew entry

[attack68]

This PR does better than existing but its still ambiguous for pathological cases - hence why I'm floating the idea of deprecating tuples as sequence identifiers for multiple keys and relying only on lists (which can't be used as index keys).

edit: actually not even doing that would solve the cases below - because nowhere is a list needed, the below depends on how the slice(None) might be automatically added or not based on implied levels.

Effect of PR

Before	After

[jbrockmendel]

hence why I'm floating the idea of deprecating tuples as sequence identifiers for multiple keys and relying only on lists (which can't be used as index keys).

I think we should do this too. This PR is about fixing the bug in the interim.

[jreback]

thanks @jbrockmendel, looking forwards to cleanups on this!