BUG: AttributeError when doing groupby with as_index=False on Empty DataFrame

[salem3358]

• 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, a copy-pastable example

# Your code here
import pandas as pd
pd.DataFrame(columns=["a", "b"]).groupby(by="a", as_index=False)["b"].sum()

Problem description

in version 0.25.3, this code return an empty DataFrame with columns=["a", "b"]

Expected Output

an empty DataFrame with columns=["a", "b"]

Output of pd.show_versions()

[paste the output of pd.show_versions() here leaving a blank line after the details tag]

INSTALLED VERSIONS

commit : None
python : 3.6.2.final.0
python-bits : 64
OS : Windows
OS-release : 10
machine : AMD64
processor : Intel64 Family 6 Model 85 Stepping 4, GenuineIntel
byteorder : little
LC_ALL : None
LANG : None
LOCALE : None.None
pandas : 1.0.5
numpy : 1.19.0
pytz : 2020.1
dateutil : 2.8.1
pip : 19.0.3
setuptools : 40.8.0
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 : None
pandas_datareader: None
bs4 : None
bottleneck : None
fastparquet : None
gcsfs : None
lxml.etree : None
matplotlib : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pytables : None
pytest : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
xlsxwriter : None
numba : None

[neil-ptr]

I'd be interested in looking into this

[jorisvandenbossche]

@neilZon that's certainly welcome! (and @salem3358 thanks for the report)

[simonjayhawkins]

the exception here is being raised by code added in #32040 (fix for #31777) i.e. 1.0.2

however, before this change, the example was still failing so the underlying issue maybe elsewhere

1.1.0.dev0+776.g9e7cb7c10
Traceback (most recent call last):
  File "C:/Users/simon/test.py", line 7, in <module>
    res = pd.DataFrame(columns=["a", "b"]).groupby(by="a", as_index=False)["b"].sum()
  File "c:\users\simon\pandas\pandas\core\groupby\groupby.py", line 1398, in func
    result = self.aggregate(lambda x: npfunc(x, axis=self.axis))
  File "c:\users\simon\pandas\pandas\core\groupby\generic.py", line 959, in aggregate
    result.columns.levels[0], name=self._selected_obj.columns.name
AttributeError: 'Index' object has no attribute 'levels'

cc @MarcoGorelli

[simonjayhawkins]

however, before this change, the example was still failing so the underlying issue maybe elsewhere

#28203 is the cause of the regression from 0.25.3

bfdbebe is the first bad commit
commit bfdbebe
Author: jbrockmendel [email protected]
Date: Fri Aug 30 11:43:48 2019 -0700

CLN: catch less inside try/except (#28203)

cc @jbrockmendel

[salem3358]

Hi guys,
Do we have any update on this issue?
Can I push a Pull request to fix this? I mean what is the procedure? I guess I will need to register to become a contributor first, right?

Thanks,
Salem

[MarcoGorelli]

@salem3358 Sure, here's the contributing guide: https://pandas.pydata.org/pandas-docs/dev/development/contributing.html

If you comment take the issue will be assigned to you. @neilZon are you still working on this? If not, then @salem3358 free to take over

[neil-ptr]

Nope, @salem3358 is free to take!

[salem3358]

take

[salem3358]

#35324