BUG: add back check for MultiIndex case and take_split_path

[jorisvandenbossche]

Closes #27841

[jorisvandenbossche]

This is reverting part of #27351: there was a seemingly unused check removed (as we thought it was for panels), but apparently it was still used (given the regression report in #27841)

There was actually discussion about this here: https://github.com/pandas-dev/pandas/pull/10838/files#r302728324

[jorisvandenbossche]

Updated.

I could simplify the case, and understand now better when this was triggered: it's when adding a single column, but where the single column selection is a slice.

In a non-MultiIndex case we actually don't allow this (and you get the same error as now in 0.25 with MultiIndex):

In [61]: df
Out[61]: 
          a         b         c
0 -2.791318 -0.238227  0.791717
1  0.635844 -1.494229  0.445973
2  0.575157  0.295088 -0.978239
3  0.894169  0.300125 -0.949338
4  0.716657 -0.634217 -2.311306

In [62]: df.loc[:, 'a':'a']
Out[62]: 
          a
0 -2.791318
1  0.635844
2  0.575157
3  0.894169
4  0.716657

In [63]: df.loc[:, 'a':'a'] = np.arange(5) 
...
ValueError: could not broadcast input array from shape (5) into shape (5,1)

But for the MultiIndex, you can get a "slice" without that the user explicitly slices the columns (as I did for the non-MultiIndex case above). So there we are more flexible.
We could maybe discuss if we want to be more strict here as well (in the future, on purpose with a deprecation), but those MultiIndex corner cases are all a bit tricky.

[jbrockmendel]

I could simplify the case, and understand now better when this was triggered: it's when adding a single column, but where the single column selection is a slice.

Simplification sounds good. If there's a reasonable scope to refactor into its own method, that would be worthwhile; the setitem_with_indexer method is a beast.

[TomAugspurger]

Merged master.

I think this is OK to fix the regression. I doubt we would want to backport any simplification code anyway.

[jbrockmendel]

Thanks @jorisvandenbossche