BUG: Behavior change in pandas 3.x with Series.resample and closed="right", how="right" #62200
Description
Pandas version checks
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I have checked that this issue has not already been reported.
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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 import pandas.testing index = pd.date_range("1-1-2000", "2-15-2000", freq="h").union(pd.date_range("4-15-2000", "5-15-2000", freq="h")) s = pd.Series(range(len(index)), index=index) left = s.resample("D", label="right", closed="right").count() right = s.resample("24h", label="right", closed="right").count() pandas.testing.assert_series_equal(left, right)Issue Description
That program passes without error with pandas==2.3.2, but fails with 3.0.0.dev0+2339.g1af0a95e94. The failure is
AssertionError: Series are different Series length are different [left]: 137, DatetimeIndex(['2000-01-01', '2000-01-02', '2000-01-03', '2000-01-04', '2000-01-05', '2000-01-06', '2000-01-07', '2000-01-08', '2000-01-09', '2000-01-10', ... '2000-05-07', '2000-05-08', '2000-05-09', '2000-05-10', '2000-05-11', '2000-05-12', '2000-05-13', '2000-05-14', '2000-05-15', '2000-05-16'], dtype='datetime64[ns]', length=137, freq='D') [right]: 136, DatetimeIndex(['2000-01-01', '2000-01-02', '2000-01-03', '2000-01-04', '2000-01-05', '2000-01-06', '2000-01-07', '2000-01-08', '2000-01-09', '2000-01-10', ... '2000-05-06', '2000-05-07', '2000-05-08', '2000-05-09', '2000-05-10', '2000-05-11', '2000-05-12', '2000-05-13', '2000-05-14', '2000-05-15'], dtype='datetime64[ns]', length=136, freq='24h') Expected Behavior
Uncertain. I think this is related to #35248 and the fix from #61985. I'm not 100% sure whether the old behavior was buggy and this was a deliberate change. But #35248 mentions "24h" and "1D" behaving differently, so perhaps this is a regresssion.
Installed Versions
INSTALLED VERSIONS ------------------ commit : 1af0a95e946bfbba814bc5b395cf65047fcac5df python : 3.12.8 python-bits : 64 OS : Darwin OS-release : 24.6.0 Version : Darwin Kernel Version 24.6.0: Mon Jul 14 11:30:40 PDT 2025; root:xnu-11417.140.69~1/RELEASE_ARM64_T6041 machine : arm64 processor : arm byteorder : little LC_ALL : None LANG : en_US.UTF-8 LOCALE : en_US.UTF-8 pandas : 3.0.0.dev0+2339.g1af0a95e94 numpy : 2.4.0.dev0+git20250808.622f874 dateutil : 2.9.0.post0 pip : None Cython : None sphinx : None IPython : 9.4.0 adbc-driver-postgresql: None adbc-driver-sqlite : None bs4 : None bottleneck : None fastparquet : None fsspec : 2025.3.2 html5lib : None hypothesis : 6.136.1 gcsfs : None jinja2 : 3.1.6 lxml.etree : None matplotlib : 3.10.3 numba : None numexpr : None odfpy : None openpyxl : None psycopg2 : None pymysql : None pyarrow : 21.0.0 pyiceberg : None pyreadstat : None pytest : 8.4.1 python-calamine : None pytz : 2025.2 pyxlsb : None s3fs : 2025.7.0 scipy : None sqlalchemy : 2.0.41 tables : None tabulate : None xarray : None xlrd : None xlsxwriter : None zstandard : None qtpy : None pyqt5 : None