Pandas different between Python 2.7 and Python 3.5

[marabout2015]

Code Sample, a copy-pastable example if possible

import pandas as pd
X = pd.DataFrame({'x': range(3)}, pd.Index(range(3), name='x'))
Y = X.x.apply(lambda x: pd.Series(range(x + 1), pd.Index(range(x + 1), name='y')))
Y.columns.names

Problem description

On Python 2.7, the above code results in:

FrozenList([u'y'])

while on Python 3.5, the same code results in:

FrozenList([None])

I see this difference between Python 2 and 3 for pandas 0.18.0, 0.19.2, and 0.20.1.

Expected Output

FrozenList([u'y'])

Or at least consistency.

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.5.2.final.0 python-bits: 64 OS: Windows OS-release: 8.1 machine: AMD64 processor: Intel64 Family 6 Model 45 Stepping 7, GenuineIntel byteorder: little LC_ALL: None LANG: None

pandas: 0.18.0
nose: 1.3.7
pip: 8.1.2
setuptools: 27.2.0
Cython: 0.24.1
numpy: 1.10.4
scipy: 0.18.1
statsmodels: 0.6.1
xarray: None
IPython: 5.1.0
sphinx: 1.4.6
patsy: 0.4.1
dateutil: 2.5.3
pytz: 2016.6.1
blosc: None
bottleneck: 1.1.0
tables: 3.2.2
numexpr: 2.6.1
matplotlib: 1.5.3
openpyxl: 2.3.2
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.3
lxml: 3.6.4
bs4: 4.5.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.13
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.42.0

[jreback]

on py3

In [9]: X = pd.DataFrame({'x': range(3)}, pd.Index(range(3), name='x'))
   ...: Y = X.x.apply(lambda x: pd.Series(range(x + 1), pd.Index(range(x + 1), name='y')))
   ...: Y.columns.names
   ...: 
Out[9]: FrozenList([None])

In [11]: X = pd.DataFrame({'x': range(3)}, pd.Index(range(3), name='x'))
    ...: Y = X.x.apply(lambda x: pd.Series(list(range(x + 1)), pd.Index(list(range(x + 1)), name='y')))
    ...: Y.columns.names
    ...: 
Out[11]: FrozenList(['y'])

These construct differently in py2 and py3 as range is a list in py2 (and an actual range object).

In [13]: Index(range(3))
Out[13]: RangeIndex(start=0, stop=3, step=1)

In [14]: Index(list(range(3)))
Out[14]: Int64Index([0, 1, 2], dtype='int64')

[jreback]

that said this path is only lightly tested. so prob not propagating the name properly. a pull-request to fix welcomed!

[lkirk]

Going to work on this issue at the pycon sprint

[topper-123]

Removing the 2/3 Compat label, as we've dropped Python2. The issue is now only that the name should probably be ['y'], though I think the use case is quite esoteric.

A PR still welcome though.

[mroeschke]

This looks to work on master. Could use a test

In [12]: import pandas as pd
    ...: X = pd.DataFrame({'x': range(3)}, pd.Index(range(3), name='x'))
    ...: Y = X.x.apply(lambda x: pd.Series(range(x + 1), pd.Index(range(x + 1), name='y')))
    ...: Y.columns.names
Out[12]: FrozenList(['y'])

In [13]: pd.__version__
Out[13]: '1.3.0.dev0+1889.gf949788596'