BUG: Incorrect behaviour when using loc on a resampled time index dataframe

[jose-moran]

Pandas version checks

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

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

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pandas as pd
df = pd.DataFrame(index=pd.date_range(start="2014-01-01", end="2023-01-01", freq="MS"))
df.resample("QS").sum().loc["2014-Q1"]

Issue Description

The example above throws a key error.
Interestingly, df.resample("QS").sum().loc["2015-Q1"] yields the indices corresponding to the first quarter of 2014.

See stackoverflow issue.

https://stackoverflow.com/questions/78315553/pandas-bug-when-trying-to-loc-data-on-a-quarter/78320367?

Expected Behavior

The view should be on the first quarter of 2014. Expected output is

Empty DataFrame
Columns: []
Index: [2014-01-01 00:00:00]

but it doesn't find it. Instead, I get the following with another indexing

In [12]: df_resampled.loc["2015-Q1"]
Out[12]:
Empty DataFrame
Columns: []
Index: [2014-01-01 00:00:00]

Note that the problem disappears if I do

df_resampled.freq = None
df_resampled.loc["2014-Q1"]

In which case the correct output is yielded.

Installed Versions

INSTALLED VERSIONS

commit : bdc79c1
python : 3.12.2.final.0
python-bits : 64
OS : Darwin
OS-release : 23.4.0
Version : Darwin Kernel Version 23.4.0: Fri Mar 15 00:12:49 PDT 2024; root:xnu-10063.101.17~1/RELEASE_ARM64_T6020
machine : arm64
processor : arm
byteorder : little
LC_ALL : None
LANG : None
LOCALE : None.UTF-8

pandas : 2.2.1
numpy : 1.26.4
pytz : 2023.3.post1
dateutil : 2.8.2
setuptools : 68.2.2
pip : 23.3.1
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : None
IPython : 8.20.0
pandas_datareader : None
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : None
bottleneck : 1.3.7
dataframe-api-compat : None
fastparquet : None
fsspec : None
gcsfs : None
matplotlib : None
numba : None
numexpr : 2.8.7
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pyreadstat : None
python-calamine : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
zstandard : None
tzdata : 2023.3
qtpy : None
pyqt5 : None

[rhshadrach]

Thanks for the report. The result of resample has freq as <QuarterBegin: startingMonth=1>. With this on the index, the lookup for "2014-Q1" is looking for Timestamp('2013-02-01 00:00:00') which seems off to me.

It doesn't appear to me that resample is the issue.

cc @MarcoGorelli

[MarcoGorelli]

thanks for the ping, adding this to my mental list of quarter oddities #8435 (comment)

[tuhinsharma121]

anyone working on it? Can I give it a try?

[tuhinsharma121]

take