Unexpected interaction in DataFrame.apply(f) when `f` returns a list #18919

gwerbin · 2017-12-23T04:20:05Z

Code Sample, a copy-pastable example if possible

df = pd.DataFrame({'x': pd.Series([['a', 'b'], ['q']]),
                   'y': pd.Series([['z'], ['q', 't']])})
df.index = pd.MultiIndex.from_tuples([('i0', 'j0'), ('i1', 'j1')])

df.apply(lambda row: [el for el in row['x'] if el in row['y']], axis=1)

Problem description

When a DataFrame has a MultiIndex, and the function passed to DataFrame.apply returns all lists, weird stuff happens and an unintelligible error occurs.

What ends up happening is that the result somehow gets coerced to a list of arrays (not sure where or why the list->array conversion happens), and then submitted to DataFrame.__init__, which tries to massage it that into a DataFrame, and fails.

Resulting error: ValueError: Empty data passed with indices specified., emitted from deep within the bowels of pandas/core/internals.py, specifically create_block_manager_from_arrays.

This happens regardless of what the reduce= argument is set to.

Expected Output

Don't try to manipulate the output. Return a Series of lists.

In the example above, that'd be:

pd.Series([[], ['q']], index=df.index)

Output of `pd.show_versions()`

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Linux
OS-release: 3.10.0-514.26.2.el7.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.21.1
pytest: None
pip: 9.0.1
setuptools: 36.6.0
Cython: None
numpy: 1.12.1
scipy: 0.19.1
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.0.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.9999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

The text was updated successfully, but these errors were encountered:

jreback · 2017-12-23T19:59:22Z

duplicate of many issues, se #18557 which will close.

closes pandas-dev#16353 closes pandas-dev#17348 closes pandas-dev#17437 closes pandas-dev#18573 closes pandas-dev#17970 closes pandas-dev#17892 closes pandas-dev#17602 closes pandas-dev#18775 closes pandas-dev#18901 closes pandas-dev#18919

closes #16353 closes #17348 closes #17437 closes #18573 closes #17970 closes #17892 closes #17602 closes #18775 closes #18901 closes #18919

…-dev#18577) closes pandas-dev#16353 closes pandas-dev#17348 closes pandas-dev#17437 closes pandas-dev#18573 closes pandas-dev#17970 closes pandas-dev#17892 closes pandas-dev#17602 closes pandas-dev#18775 closes pandas-dev#18901 closes pandas-dev#18919

jreback closed this as completed Dec 23, 2017

jreback mentioned this issue Dec 23, 2017

API/BUG: .apply will correctly infer output shape when axis=1 #18577

Merged

jreback added Apply Apply, Aggregate, Transform Reshaping Concat, Merge/Join, Stack/Unstack, Explode labels Dec 23, 2017

jreback added this to the No action milestone Dec 23, 2017

jreback added the Duplicate Report Duplicate issue or pull request label Dec 23, 2017

jorisvandenbossche modified the milestones: No action, 0.23.0 Jan 28, 2018

jorisvandenbossche pushed a commit that referenced this issue Feb 7, 2018

API/BUG: .apply will correctly infer output shape when axis=1 (#18577)

6b0c7e7

closes #16353 closes #17348 closes #17437 closes #18573 closes #17970 closes #17892 closes #17602 closes #18775 closes #18901 closes #18919

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Unexpected interaction in DataFrame.apply(f) when `f` returns a list #18919

Unexpected interaction in DataFrame.apply(f) when `f` returns a list #18919

gwerbin commented Dec 23, 2017

jreback commented Dec 23, 2017

Unexpected interaction in DataFrame.apply(f) when f returns a list #18919

Unexpected interaction in DataFrame.apply(f) when f returns a list #18919

Comments

gwerbin commented Dec 23, 2017

Code Sample, a copy-pastable example if possible

Problem description

Expected Output

Output of pd.show_versions()

jreback commented Dec 23, 2017

Unexpected interaction in DataFrame.apply(f) when `f` returns a list #18919

Unexpected interaction in DataFrame.apply(f) when `f` returns a list #18919

Output of `pd.show_versions()`