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Reproducible Example

import pandas as pd
series = pd.Series([5, 7], index=['b', 'a'])
assert list(series.groupby(level=0).groups.keys()) == list(series.groupby(pd.Grouper(level=0)).groups.keys())

Issue Description

series.groupby(level=0) and series.groupby(pd.Grouper(level=0)) produce different orders

Expected Behavior

The result of series.groupby(level=0) should be sorted just as the result of series.groupby(pd.Grouper(level=0)) is.

Installed Versions

INSTALLED VERSIONS ------------------ commit : c888af6d0bb674932007623c0867e1fbd4bdc2c6 python : 3.13.5 python-bits : 64 OS : Darwin OS-release : 24.5.0 Version : Darwin Kernel Version 24.5.0: Tue Apr 22 19:54:25 PDT 2025; root:xnu-11417.121.6~2/RELEASE_ARM64_T6020 machine : arm64 processor : arm byteorder : little LC_ALL : None LANG : en_US.UTF-8 LOCALE : en_US.UTF-8 pandas : 2.3.1 numpy : 2.3.2 pytz : 2025.2 dateutil : 2.9.0.post0 pip : 25.1 Cython : None sphinx : None IPython : 9.4.0 adbc-driver-postgresql: None adbc-driver-sqlite : None bs4 : None blosc : None bottleneck : None dataframe-api-compat : None fastparquet : None fsspec : None html5lib : None hypothesis : None gcsfs : None jinja2 : None lxml.etree : None matplotlib : None numba : None numexpr : None odfpy : None openpyxl : None pandas_gbq : None psycopg2 : None pymysql : None pyarrow : None pyreadstat : None pytest : None python-calamine : None pyxlsb : None s3fs : None scipy : None sqlalchemy : None tables : None tabulate : None xarray : None xlrd : None xlsxwriter : None zstandard : None tzdata : 2025.2 qtpy : None pyqt5 : None

Comment From: geekalaa

The issue is caused by inconsistent default values for the sort parameter:

  • pd.Grouper.init has sort: bool = False
  • Series.groupby has sort: bool = True

This explains why: - series.groupby(level=0) returns keys in sorted order: ['a', 'b'] (uses sort=True default) - series.groupby(pd.Grouper(level=0)) returns keys in original index order: ['b', 'a'] (uses sort=False default)

Comment From: rhshadrach

Thanks for the report. I do think we should align these to default to sort=True. @jbrockmendel - do you agree?

Comment From: jbrockmendel

agreed i would expect these to match, no real preference on what the default should be.

Comment From: rhshadrach

Main reason I would change Grouper is that I'd guess it sees a lot less use.