QST: Why is indexing with a reversed list possible but not assigning a new value?
- [ x] I have searched the [[pandas] tag](https://stackoverflow.com/questions/tagged/pandas) on StackOverflow for similar questions.

- [ ] I have asked my usage related question on [StackOverflow](https://stackoverflow.com).

---

#### Question about pandas

I noticed that indexing with a reversed list works but not assigning. The error message is not helpful. Is this intended?

```python
import pandas as pd

s = pd.Series([1, 2, 3])
mask = [True, False, False]
s.loc[reversed(mask)]
>>> 2    3
>>> dtype: int64

# But assigning does not work
s.loc[reversed(mask)] = 5
>>> ---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-1-912755dd04b5> in <module>
      7 
      8 # But assigning does not work
----> 9 s.loc[reversed(mask)] = 5

~/miniconda3/envs/who/lib/python3.9/site-packages/pandas/core/indexing.py in __setitem__(self, key, value)
    686         else:
    687             key = com.apply_if_callable(key, self.obj)
--> 688         indexer = self._get_setitem_indexer(key)
    689         self._has_valid_setitem_indexer(key)
    690 

~/miniconda3/envs/who/lib/python3.9/site-packages/pandas/core/indexing.py in _get_setitem_indexer(self, key)
    634 
    635         try:
--> 636             return self._convert_to_indexer(key, axis=0, is_setter=True)
    637         except TypeError as e:
    638 

~/miniconda3/envs/who/lib/python3.9/site-packages/pandas/core/indexing.py in _convert_to_indexer(self, key, axis, is_setter)
   1210             else:
   1211                 # When setting, missing keys are not allowed, even with .loc:
-> 1212                 return self._get_listlike_indexer(key, axis, raise_missing=True)[1]
   1213         else:
   1214             try:

~/miniconda3/envs/who/lib/python3.9/site-packages/pandas/core/indexing.py in _get_listlike_indexer(self, key, axis, raise_missing)
   1264             keyarr, indexer, new_indexer = ax._reindex_non_unique(keyarr)
   1265 
-> 1266         self._validate_read_indexer(keyarr, indexer, axis, raise_missing=raise_missing)
   1267         return keyarr, indexer
   1268 

~/miniconda3/envs/who/lib/python3.9/site-packages/pandas/core/indexing.py in _validate_read_indexer(self, key, indexer, axis, raise_missing)
   1306             if missing == len(indexer):
   1307                 axis_name = self.obj._get_axis_name(axis)
-> 1308                 raise KeyError(f"None of [{key}] are in the [{axis_name}]")
   1309 
   1310             ax = self.obj._get_axis(axis)

KeyError: "None of [Index([False, False, True], dtype='object')] are in the [index]"
```
pd.show_version() outputs this:
INSTALLED VERSIONS
------------------
commit           : 9d598a5e1eee26df95b3910e3f2934890d062caa
python           : 3.9.1.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.19.128-microsoft-standard
Version          : #1 SMP Tue Jun 23 12:58:10 UTC 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.1
numpy            : 1.19.2
pytz             : 2020.5
dateutil         : 2.8.1
pip              : 20.3.3
setuptools       : 52.0.0.post20210125
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.19.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.2
numexpr          : None
odfpy            : None
openpyxl         : 3.0.6
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.2
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None
