BUG: Index alignment behaviour

[complexsum]

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• (optional) I have confirmed this bug exists on the master branch of pandas.

---

Note: Please read this guide detailing how to provide the necessary information for us to reproduce your bug.

Code Sample

np.random.seed(1)
df = pd.DataFrame(np.random.randint(0, 100, (10, 2)), columns=['A', 'B'])

>>> df
    A   B
0  37  12
1  72   9
2  75   5
3  79  64
4  16   1
5  76  71
6   6  25
7  50  20
8  18  84
9  11  28

mask = df['A'] >= 40
>>> df[mask]
    A   B
1  72   9
2  75   5
3  79  64
5  76  71
7  50  20

>>> df[mask].sort_values('A')
    A   B
7  50  20
1  72   9
2  75   5
5  76  71
3  79  64

CASE 1: Assignment with .loc

No problem here this is expected behaviour as pandas align data on indices.

df.loc[mask] = df[mask].sort_values('A')
>>> df[mask] 

    A   B
1  72   9
2  75   5
3  79  64
5  76  71
7  50  20

CASE 2: Assignment without .loc

The problem is here. In pandas version 1.2.x assignment without using .loc no longer respects index alignment behaviour which was previously respected in version 1.1.x.

df[mask] = df[mask].sort_values('A')
>>> df[mask]

    A   B
1  50  20
2  72   9
3  75   5
5  76  71
7  79  64

Problem description

Lets say we want to sort the values of the slice of the dataframe and assign the sorted values back to the original dataframe in-place.

• In CASE 1 when using assignment with .loc no effect of sorting the slice is observed in original dataframe which is expected as pandas align data on indices.
• In CASE 2 when using assignment without .loc the assignment operation no longer respects the alignment of indices which is unexpected behaviour in pandas version 1.2.x

Prior to pandas version 1.2.x lets say in pandas version 1.1.5 the assignment operation even without using the loc respected the index alignment behaviour.

Expected Output

In CASE 2 the expected output should be:

>>> df[mask]
    A   B
1  72   9
2  75   5
3  79  64
5  76  71
7  50  20

As the assignment operation should respect the pandas alignment of indices behaviour.

Output of pd.show_versions()

INSTALLED VERSIONS

commit : 7d32926
python : 3.8.6.final.0
python-bits : 64
OS : Linux
OS-release : 5.8.0-44-generic
Version : #50-Ubuntu SMP Tue Feb 9 06:29:41 UTC 2021
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_IN
LOCALE : en_IN.ISO8859-1

pandas : 1.2.2
numpy : 1.18.5
pytz : 2020.1
dateutil : 2.8.1
pip : 20.1.1
setuptools : 44.0.0
Cython : None
pytest : 6.2.1
hypothesis : None
sphinx : 3.4.3
blosc : None
feather : None
xlsxwriter : 1.3.5
lxml.etree : 4.6.2
html5lib : 1.1
pymysql : None
psycopg2 : None
jinja2 : 2.11.2
IPython : 7.18.1
pandas_datareader: 0.9.0
bs4 : 4.9.1
bottleneck : None
fsspec : 0.8.5
fastparquet : 0.5.0
gcsfs : None
matplotlib : 3.3.2
numexpr : None
odfpy : None
openpyxl : 3.0.5
pandas_gbq : None
pyarrow : 2.0.0
pyxlsb : None
s3fs : None
scipy : 1.5.2
sqlalchemy : 1.3.22
tables : None
tabulate : 0.8.7
xarray : 0.16.2
xlrd : 1.2.0
xlwt : None
numba : 0.50.1

[phofl]

Context: We are dispatching to iloc under the hood. Iloc stopped aligning in 1.2

[rhshadrach]

@phofl - It seems to me iloc not aligning is the correct behavior, but this induced a regression in loc not aligning - is that accurate?

[phofl]

DataFrame.setitem does not align, loc behaves fine, but yes this is a regression

[rhshadrach]

Ah thanks - makes sense. I got the two cases in the OP mixed up.

[simonjayhawkins]

Context: We are dispatching to iloc under the hood. Iloc stopped aligning in 1.2

first bad commit: [3d259e6] Bug in iloc aligned objects (#37728)