BUG: pd.DataFrame.transform recursively loops in some cases

[adamczykm]

• 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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Note: Please read this guide detailing how to provide the necessary information for us to reproduce your bug.

Code Sample, a copy-pastable example

pd.DataFrame({"a":[None]}).transform({"a":int})

Problem description

Executing the above causes recursion depth limit exception. This is confusing and it is harder to pinpoint/debug than the expected exception.

Expected Output

Something akin to the output of int(None)

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit : None python : 3.7.5.final.0 python-bits : 64 OS : Darwin OS-release : 18.6.0 machine : x86_64 processor : i386 byteorder : little LC_ALL : None LANG : None LOCALE : None.UTF-8

pandas : 1.0.3
numpy : 1.18.4
pytz : 2020.1
dateutil : 2.8.1
pip : 19.2.3
setuptools : 41.2.0
Cython : None
pytest : 3.10.1
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 : 3.10.1
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
xlsxwriter : None
numba : None

[charlesdong1991]

thanks for the report, and would be nicer to have a descriptive issue title!

[pedrooa]

hi, can i take this?

[charlesdong1991]

sure, go ahead! @pedrooa

[dsaxton]

@pedrooa Looks like the problem is the the try / except within aggregate of frame.py, would be interesting to look into why that's needed and if there's a better way to handle things

[pedrooa]

I've looked into the try / except you mentioned(within aggregate of frame.py), and the recurssion occurs because the except is not doing anything with the error:

try:
    result, how = self._aggregate(func, axis=axis, *args, **kwargs)
except TypeError:
    pass

I've managed to fix it raising an error printing what went wrong, as in the following code:

try:
    result, how = self._aggregate(func, axis=axis, *args, **kwargs)
except TypeError as err:
    exc = TypeError(
                    "DataFrame constructor called with "
                    f"incompatible data and dtype: {err}"
                )
    raise exc from err

I am now just making sure this fix doesn't break anything else.