BUG: SeriesGroupBy.value_counts() throws IndexError if there is only one group

[sleiken]

• 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.

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Code Sample, a copy-pastable example

import pandas as pd
pd.DataFrame([[0, 1]], columns=('a', 'b')).groupby('a')['b'].value_counts()

IndexError                                Traceback (most recent call last)
<ipython-input-53-ab16326ac16f> in <module>
----> 1 pd.DataFrame([[0,1]], columns=('a', 'b')).groupby('a')['b'].value_counts()

~/.venvs/python3/lib/python3.9/site-packages/pandas/core/groupby/generic.py in value_counts(self, normalize, sort, ascending, bins, dropna)
    762         if not len(lchanges):
    763             inc = lchanges
--> 764         inc[idx] = True  # group boundaries are also new values
    765         out = np.diff(np.nonzero(np.r_[inc, True])[0])  # value counts
    766

IndexError: index 0 is out of bounds for axis 0 with size 0

Problem description

Calling value_counts() on a SeriesGroupBy throws an IndexError when there is only one group.

Expected Output

a  b
0  1    1
Name: b, dtype: int64

Output of pd.show_versions()

INSTALLED VERSIONS

commit : f00ed8f
python : 3.9.4.final.0
python-bits : 64
OS : Darwin
OS-release : 19.6.0
Version : Darwin Kernel Version 19.6.0: Mon Apr 12 20:57:45 PDT 2021; root:xnu-6153.141.28.1~1/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.3.0
numpy : 1.20.2
pytz : 2021.1
dateutil : 2.8.1
pip : 21.0.1
setuptools : 54.2.0
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 : 7.23.0
pandas_datareader: None
bs4 : None
bottleneck : None
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 4.0.0
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
numba : None

[sleiken]

Correction: error occurs when there is only one row in the dataframe that the grouby is created from not when there is one group.

[neelmraman]

take

[rhshadrach]

Thanks for the report! Verified on master.

[neelmraman]

@rhshadrach mind also taking a look at the PR when you get the chance? thanks!