BUG: Inconsistency when empty inputs are passed to agg for groupby

[ntachukwu]

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(data={'a': [1, 2, 3], 'b': [4, 5, 6]})
>>> print(df.groupby("a").agg([])) # Passing an empty list as argument
Empty DataFrame
Columns: []
Index: []
>>> print(df.groupby("a").agg(())) # Passing an empty tuple as argument
Empty DataFrame
Columns: []
Index: []
>>> print(df.groupby("a").agg({})) # Passing an empty dictionary as argument
....
ValueError: No objects to concatenate

Issue Description

Passing an empty list or tuple to groupby.agg returns an empty dataframe but an empty dictionary raises a ValueError.

This is part of an effort to find consistency between pandas operations with empty inputs. Issue #47959 is an attempt to start a discussion on this.

Expected Behavior

Some Consistency

Installed Versions

INSTALLED VERSIONS ------------------ commit : 519fa10 python : 3.9.10.final.0 python-bits : 64 OS : Darwin OS-release : 21.3.0 Version : Darwin Kernel Version 21.3.0: Wed Jan 5 21:37:58 PST 2022; root:xnu-8019.80.24~20/RELEASE_ARM64_T8101 machine : arm64 processor : arm byteorder : little LC_ALL : None LANG : None LOCALE : None.UTF-8

pandas : 1.5.0.dev0+1478.g94044c8532
numpy : 1.22.3
pytz : 2022.1
dateutil : 2.8.2
setuptools : 60.9.3
pip : 22.1.1
Cython : 0.29.32
pytest : 7.1.2
hypothesis : 6.52.3
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.8.0
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.1.2
IPython : 8.3.0
pandas_datareader: 0.10.0
bs4 : 4.11.1
bottleneck : None
brotli : None
fastparquet : None
fsspec : None
gcsfs : None
matplotlib : 3.5.2
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : 1.8.1
snappy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
zstandard : None
tzdata : None

[eshanja1n]

take

[topper-123]

Thanks for this report, @Th3nn3ss.

I agree that these should behave similarly for consistency. I'm not super sure however ig I think that these should work or whether we should instead raise if given an empty list/dict. @rhshadrach, do you have an opinion?

If we do accept empty lists/dicts IMO it would be most logical to still keep the groupings, in the examples above return

Empty DataFrame
Columns: []
Index: [1, 2, 3]

That is, always return the same index, even if called with empty lists/dicts.

[rhshadrach]

+1 on not raising here, and returning the frame @topper-123 suggested.