BUG: pandas-1.4.0 .loc "shallow" copies do not update #45743
Description
Pandas version checks
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I have confirmed this bug exists on the latest version of pandas.
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I have confirmed this bug exists on the main branch of pandas.
Reproducible Example
import pandas as pd from numpy.random import default_rng rng = default_rng(20220125) N = 10 original = pd.DataFrame({'a': rng.standard_normal(N), 'b': rng.integers(low=0, high=100, size=N), }) print(f'Pandas Version : {pd.__version__}') print(f'===========================') print(f'Original :\n{original}\n') copy_plain = original.loc[1:2] copy_shallow = original.loc[1:2].copy(deep=False) copy_deep = original.loc[1:2].copy(deep=True) def print_df(df, description): """Check the data frame.""" print(f'===========================') print(f'{description} : \n{df}') print_df(original.loc[1:2], 'Original slice') print_df(copy_plain, 'Plain') print_df(copy_shallow, 'Shallow') print_df(copy_deep, 'Deep') print(f'Make a change to original and see if it cascades through') print(f'original["a"] = original["a"]**2') original['a'] = original['a']**2 print_df(original.loc[1:2], 'Original slice') print_df(copy_plain, 'Plain') print_df(copy_shallow, 'Shallow') print_df(copy_deep, 'Deep')Issue Description
When taking a "shallow" slice of a dataframe with .loc subsequent updates should be reflected in the original and vice-versa, this is no longer the case under Pandas 1.4.0 as demonstrated running the reproducible example...
Pandas Version : 1.4.0 =========================== Original : a b 0 0.778724 58 1 1.529873 40 2 -0.328487 63 3 -0.090816 66 4 0.451966 82 5 1.648603 65 6 1.161088 92 7 0.058186 48 8 0.800209 81 9 1.783765 4 =========================== Original slice : a b 1 1.529873 40 2 -0.328487 63 =========================== Plain : a b 1 1.529873 40 2 -0.328487 63 =========================== Shallow : a b 1 1.529873 40 2 -0.328487 63 =========================== Deep : a b 1 1.529873 40 2 -0.328487 63 Make a change to original and see if it cascades through original["a"] = original["a"]**2 =========================== Original slice : a b 1 2.340512 40 2 0.107903 63 =========================== Plain : a b 1 1.529873 40 2 -0.328487 63 =========================== Shallow : a b 1 1.529873 40 2 -0.328487 63 =========================== Deep : a b 1 1.529873 40 2 -0.328487 63 After original["a"] = original["a"]**2 the values at .loc[1:2,"a"]are2.340512and0.107903 and should be the same under a "Plain" copy and "Shallow`" copy as the documentation states only a "deep" copy should take a snapshot of the original and shallow copies share data and index with original..
Expected Behavior
The behaviour under 1.3.5 is as described in the manual, after modifying the original a "plain" and "shallow" copy reflect the modifications...
Pandas Version : 1.3.5 =========================== Original : a b 0 0.778724 58 1 1.529873 40 2 -0.328487 63 3 -0.090816 66 4 0.451966 82 5 1.648603 65 6 1.161088 92 7 0.058186 48 8 0.800209 81 9 1.783765 4 =========================== Original slice : a b 1 1.529873 40 2 -0.328487 63 =========================== Plain : a b 1 1.529873 40 2 -0.328487 63 =========================== Shallow : a b 1 1.529873 40 2 -0.328487 63 =========================== Deep : a b 1 1.529873 40 2 -0.328487 63 Make a change to original and see if it cascades through original["a"] = original["a"]**2 =========================== Original slice : a b 1 2.340512 40 2 0.107903 63 =========================== Plain : a b 1 2.340512 40 2 0.107903 63 =========================== Shallow : a b 1 2.340512 40 2 0.107903 63 =========================== Deep : a b 1 1.529873 40 2 -0.328487 63 Installed Versions
INSTALLED VERSIONS
commit : bb1f651
python : 3.8.12.final.0
python-bits : 64
OS : Linux
OS-release : 4.15.0-123-generic
Version : #126-Ubuntu SMP Wed Oct 21 09:40:11 UTC 2020
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_GB.UTF-8
LOCALE : en_GB.UTF-8
pandas : 1.4.0
numpy : 1.22.1
pytz : 2021.3
dateutil : 2.8.2
pip : 21.3.1
setuptools : 60.5.0
Cython : 0.29.26
pytest : 6.2.5
hypothesis : None
sphinx : 4.4.0
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.7.1
html5lib : None
pymysql : None
psycopg2 : 2.9.3
jinja2 : 3.0.3
IPython : 8.0.1
pandas_datareader: None
bs4 : 4.10.0
bottleneck : None
fastparquet : None
fsspec : 2022.01.0
gcsfs : None
matplotlib : 3.5.1
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : 1.7.3
sqlalchemy : 1.4.31
tables : None
tabulate : 0.8.9
xarray : 0.20.2
xlrd : 2.0.1
xlwt : None
zstandard : None