BUG: Pandas 1.0.5 → 1.1.0 behavior change on DataFrame.apply() where fn returns np.ndarray

[dechamps]

• 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.

---

Code Sample, a copy-pastable example

print(type(
  pd.DataFrame([['foo']])
  .apply(lambda col: np.array('bar'))
  .iloc[0]))

Output of Pandas 1.0.5

<class 'str'>

Output of Pandas 1.1.0

<class 'numpy.ndarray'>

It is not clear to me if this behaviour change is intended or not. I couldn't find anything obvious in the release notes.

I discovered it because it broke my code, which returns the output of DataFrame.unique().squeeze() (with the intent of extracting a scalar) in an apply() func. Arguably my code is wrong, because DataFrame.unique() returns an np.ndarray, and calling squeeze() on that always returns an ndarray, never a scalar - the correct call would have been item(). Up until now, Pandas would "hide" the bug because it would take the one-element ndarray and treat it as a scalar. Not anymore, it seems. My code ultimately fails downstream of the call as it tries to hash the resulting string, which doesn't work if the string is actually a 1-element ndarray.

Output of pd.show_versions()

INSTALLED VERSIONS

commit : d9fff27
python : 3.6.9.final.0
python-bits : 64
OS : Linux
OS-release : 4.19.104+
Version : #1 SMP Wed Feb 19 05:26:34 PST 2020
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.1.0
numpy : 1.19.1
pytz : 2018.9
dateutil : 2.8.1
pip : 19.3.1
setuptools : 49.2.0
Cython : 0.29.21
pytest : 3.6.4
hypothesis : None
sphinx : 1.8.5
blosc : None
feather : 0.4.1
xlsxwriter : None
lxml.etree : 4.2.6
html5lib : 1.0.1
pymysql : None
psycopg2 : 2.7.6.1 (dt dec pq3 ext lo64)
jinja2 : 2.11.2
IPython : 5.5.0
pandas_datareader: None
bs4 : 4.6.3
bottleneck : 1.3.2
fsspec : 0.7.4
fastparquet : None
gcsfs : None
matplotlib : 3.2.2
numexpr : 2.7.1
odfpy : None
openpyxl : 2.5.9
pandas_gbq : 0.11.0
pyarrow : 0.14.1
pytables : None
pyxlsb : None
s3fs : 0.4.2
scipy : 1.4.1
sqlalchemy : 1.3.18
tables : 3.4.4
tabulate : 0.8.7
xarray : 0.15.1
xlrd : 1.1.0
xlwt : 1.3.0
numba : 0.48.0

[simonjayhawkins]

Thanks @dechamps for the report. As this appears to be an undocumented change in behaviour, i'll mark as a regression for now pending further discussion/investigation

[simonjayhawkins]

The commit that caused this change is #34909. (This also caused another regression #35462) cc @jbrockmendel

91802a9 is the first bad commit
commit 91802a9
Author: jbrockmendel jbrockmendel@gmail.com
Date: Thu Jun 25 16:06:10 2020 -0700

PERF: avoid creating many Series in apply_standard (#34909)

[dechamps]

I also just found #35518 which is somewhat similar in that it concerns values returned from the apply func being interpreted in a different way in Pandas 1.1.0.

[jbrockmendel]

If anything the 1.1.0 behavior looks more correct to me.