QST: Why is indexing with a reversed list possible but not assigning a new value?

[aauss]

• [ x] I have searched the [pandas] tag on StackOverflow for similar questions.

• I have asked my usage related question on StackOverflow.

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Question about pandas

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

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 : 9d598a5
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

[MarcoGorelli]

Hi @aauss - can you post the full traceback, as well as pd.show_versions() (as per the issue template)?

[aauss]

Hi @MarcoGorelli, I just updated my issue to contain the traceback and pd.show_version().

Apologies for missing that!

[phofl]

This is a bug, will make a PR to fix this tomorrow