BUG: groupby / resample / aggregate produces wrong results

[roy2006]

• 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

data = """        date  revenue name
0 2020-02-24        9  n_1
1 2020-05-12        8  n_2
2 2020-03-28        9  n_2
3 2020-01-14        2  n_0"""

df = pd.read_csv(StringIO(data), sep = "\s+", engine = "python")
df.date = pd.to_datetime(df.date)     

gb = df.groupby("name").resample("M", on="date")

res1 = gb.sum()[['revenue']]

# ==>
#                  revenue
# name date               
# n_0  2020-01-31        2
# n_1  2020-02-29        9
# n_2  2020-03-31        9
#      2020-04-30        0
#      2020-05-31        8

res2 = gb.aggregate({'revenue':'sum'})

# ==>
#                  revenue
# name date               
# n_0  2020-05-31        2
# n_1  2020-01-31        9
# n_2  2020-02-29        8
#      2020-03-31        9

Problem description

As the code sample above shows, there are two issues:

• agg(sum) produces different results from just sum.
• The results of agg(sum) are plain wrong. The name "n_2" has no data in Feb, and yet - the aggregation shows that it does.

Expected Output

The correct output is the one produced by sum.

Output of pd.show_versions()

INSTALLED VERSIONS

commit : None
python : 3.7.5.final.0
python-bits : 64
OS : Darwin
OS-release : 19.5.0
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.0.5
numpy : 1.17.4
pytz : 2019.3
dateutil : 2.8.0
pip : 19.3.1
setuptools : 41.6.0
Cython : None
pytest : 5.4.3
hypothesis : 5.18.0
sphinx : None
blosc : None
feather : None
xlsxwriter : 1.2.9
lxml.etree : 4.5.0
html5lib : 1.0.1
pymysql : None
psycopg2 : 2.8.4 (dt dec pq3 ext lo64)
jinja2 : 2.10.3
IPython : 7.10.1
pandas_datareader: 0.8.1
bs4 : 4.9.0
bottleneck : None
fastparquet : None
gcsfs : None
lxml.etree : 4.5.0
matplotlib : 3.2.1
numexpr : None
odfpy : None
openpyxl : 3.0.3
pandas_gbq : None
pyarrow : None
pytables : None
pytest : 5.4.3
pyxlsb : None
s3fs : None
scipy : 1.4.1
sqlalchemy : 1.3.16
tables : None
tabulate : None
xarray : 0.15.1
xlrd : 1.2.0
xlwt : None
xlsxwriter : 1.2.9
numba : None

[MarcoGorelli]

Thanks @roy2006 for the excellent bug report!

[simonjayhawkins]

Thanks @roy2006 this looks like a duplicate of #33548 so closing. lmk if i'm overlooking something.