QST: Why is indexing with a reversed list possible but not assigning a new value? · Issue #39614 · pandas-dev/pandas (original) (raw)

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) in 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