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Showing content from https://github.com/pandas-dev/pandas/issues/48481 below:

Support "std" aggregation in groupby for groupby datetime column · Issue #48481 · pandas-dev/pandas · GitHub

• 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.

import pandas as pd

df = pd.DataFrame(
    [
        [1, "2022-02-02"],
        [1, "2022-03-02"],
        [1, "2022-03-02"],
        [2, "2022-04-02"],
        [2, "2022-04-02"],
        [2, "2022-05-02"],
    ], columns=["a", "dt"]
)
df["dt"] = pd.to_datetime(df["dt"])
gb = df.groupby("a")
print(gb["dt"].agg(lambda d: d.std()))
print(gb["dt"].agg("std"))

Computing standard deviation fails on GroupBy object, but it is successful with the std method on DataFrame.

Traceback (most recent call last):
  File "example.py", line 16, in <module>
    print(gb["dt"].agg("std"))
  File "pandas/pandas/core/groupby/generic.py", line 276, in aggregate
    return getattr(self, func)(*args, **kwargs)
  File "pandas/pandas/core/groupby/groupby.py", line 2261, in std
    result = self._get_cythonized_result(
  File "pandas/pandas/core/groupby/groupby.py", line 3818, in _get_cythonized_result
    res_mgr = mgr.grouped_reduce(blk_func, ignore_failures=True)
  File "pandas/core/internals/base.py", line 199, in grouped_reduce
    res = func(arr)
  File "pandas/pandas/core/groupby/groupby.py", line 3778, in blk_func
    vals = vals.astype(cython_dtype, copy=False)
  File "pandas/pandas/core/arrays/datetimes.py", line 636, in astype
    return dtl.DatetimeLikeArrayMixin.astype(self, dtype, copy)
  File "pandas/pandas/core/arrays/datetimelike.py", line 507, in astype
    raise TypeError(msg)
TypeError: Cannot cast DatetimeArray to dtype float64

Computing standard deviation on GroupBy should be successful.

Tested on master and released version. Failed in both cases.

commit : 047c11d
python : 3.10.4.final.0
python-bits : 64
OS : Darwin
OS-release : 21.6.0
Version : Darwin Kernel Version 21.6.0: Wed Aug 10 14:28:23 PDT 2022; root:xnu-8020.141.5~2/RELEASE_ARM64_T6000
machine : arm64
processor : arm
byteorder : little
LC_ALL : None
LANG : None
LOCALE : None.UTF-8
pandas : 1.6.0.dev0+89.g047c11d780
numpy : 1.22.4
pytz : 2022.1
dateutil : 2.8.2
setuptools : 61.2.0
pip : 22.1.2
Cython : None
pytest : None
hypothesis : None
sphinx : 5.1.1
blosc : None
feather : None
xlsxwriter : 3.0.3
lxml.etree : 4.9.1
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.1.2
IPython : 8.4.0
pandas_datareader: None
bs4 : 4.11.1
bottleneck : 1.3.5
brotli : None
fastparquet : None
fsspec : None
gcsfs : None
matplotlib : 3.5.2
numba : 0.56.0
numexpr : None
odfpy : None
openpyxl : 3.0.10
pandas_gbq : None
pyarrow : None
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : 1.8.1
snappy : None
sqlalchemy : 1.4.39
tables : None
tabulate : 0.8.10
xarray : None
xlrd : 2.0.1
xlwt : None
zstandard : None
tzdata : None

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