Make MultiIndex.get_loc raise for unhashable type

[dsaxton]

• closes BUG: MultiIndex assignment via loc #35878
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[dsaxton]

I think it is okay to remove this comment since we're not actually checking for list here, or should it be moved? cc @jbrockmendel

[jbrockmendel]

i think youre right (though havent checked 30892)

[jbrockmendel]

should this raise InvalidIndexError?

[dsaxton]

That was my original thought, but it broke an existing test because evidently passing a singleton list with only np.nan raises the unhashable type error (it hits a line that uses this approach of trying to hash the key):

import numpy as np
import pandas as pd

idx = pd.MultiIndex.from_tuples([("a", 1), ("b", 2)])
idx.get_loc([np.nan])

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-6-40dd73c454ae> in <module>
----> 1 idx.get_loc([np.nan])

~/opt/miniconda3/envs/pandas-1.1.1/lib/python3.8/site-packages/pandas/core/indexes/multi.py in get_loc(self, key, method)
   2717         lead_key, follow_key = key[:i], key[i:]
   2718         start, stop = (
-> 2719             self.slice_locs(lead_key, lead_key) if lead_key else (0, len(self))
   2720         )
   2721

~/opt/miniconda3/envs/pandas-1.1.1/lib/python3.8/site-packages/pandas/core/indexes/multi.py in slice_locs(self, start, end, step, kind)
   2578         # This function adds nothing to its parent implementation (the magic
   2579         # happens in get_slice_bound method), but it adds meaningful doc.
-> 2580         return super().slice_locs(start, end, step, kind=kind)
   2581
   2582     def _partial_tup_index(self, tup, side="left"):

~/opt/miniconda3/envs/pandas-1.1.1/lib/python3.8/site-packages/pandas/core/indexes/base.py in slice_locs(self, start, end, step, kind)
   5159         start_slice = None
   5160         if start is not None:
-> 5161             start_slice = self.get_slice_bound(start, "left", kind)
   5162         if start_slice is None:
   5163             start_slice = 0

~/opt/miniconda3/envs/pandas-1.1.1/lib/python3.8/site-packages/pandas/core/indexes/multi.py in get_slice_bound(self, label, side, kind)
   2522         if not isinstance(label, tuple):
   2523             label = (label,)
-> 2524         return self._partial_tup_index(label, side=side)
   2525
   2526     def slice_locs(self, start=None, end=None, step=None, kind=None):

~/opt/miniconda3/envs/pandas-1.1.1/lib/python3.8/site-packages/pandas/core/indexes/multi.py in _partial_tup_index(self, tup, side)
   2593             section = labs[start:end]
   2594
-> 2595             if lab not in lev and not isna(lab):
   2596                 if not lev.is_type_compatible(lib.infer_dtype([lab], skipna=False)):
   2597                     raise TypeError(f"Level type mismatch: {lab}")

~/opt/miniconda3/envs/pandas-1.1.1/lib/python3.8/site-packages/pandas/core/indexes/base.py in __contains__(self, key)
   4063         False
   4064         """
-> 4065         hash(key)
   4066         try:
   4067             return key in self._engine

TypeError: unhashable type: 'list'

[jreback]

lgtm merge on green.

[simonjayhawkins]

@meeseeksdev backport 1.1.x