DataFrame.apply() with function that return category series

[ruoyu0088]

import pandas as pd
df = pd.DataFrame({"c0":["A","A","B","B"], "c1":["C","C","D","D"]})
df.apply(lambda s:s.astype("category"))

the resut is a series with series as element, not a dataframe:

c0    [A, A, B, B]
Categories (2, object): [A < B]
c1    [C, C, D, D]
Categories (2, object): [C < D]
dtype: object

Here is the output of `show_vershons()`:

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.9.final.0
python-bits: 32
OS: Windows
OS-release: 7
machine: x86
processor: x86 Family 6 Model 42 Stepping 7, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.15.2.dev
nose: 1.3.4
Cython: 0.21.2
numpy: 1.9.1
scipy: 0.15.0
statsmodels: 0.6.1
IPython: 3.0.0
sphinx: 1.2.3
patsy: 0.3.0
dateutil: 2.3
pytz: 2014.10
bottleneck: None
tables: 3.1.1
numexpr: 2.4
matplotlib: 1.4.2
openpyxl: None
xlrd: 0.9.3
xlwt: None
xlsxwriter: 0.6.5
lxml: 3.4.1
bs4: 4.3.2
html5lib: None
httplib2: None
apiclient: None
rpy2: 2.5.4
sqlalchemy: 0.9.8
pymysql: None
psycopg2: None

[jreback]

hmm, not behaving properly. as a work-around you can do this:

In [8]: DataFrame(dict([(c,col.astype('category')) for c, col in df.iteritems()]))
Out[8]: 
  c0 c1
0  A  C
1  A  C
2  B  D
3  B  D

In [9]: DataFrame(dict([(c,col.astype('category')) for c, col in df.iteritems()])).dtypes
Out[9]: 
c0    category
c1    category
dtype: object

[sebp]

This problem is still present in 0.16.0, apply does not retain category dtypes.

import pandas

df = pandas.DataFrame({'col0': [1, 2, 3, 4, 5],
                       'col1': ['yes', 'no', 'no', 'yes', 'yes'],
                       'col2': ['small', 'large', 'medium', 'large', 'small']})
df['col2'] = pandas.Categorical(df['col2'], categories=['small', 'medium', 'large'],
                                ordered=True)

other = df.apply(lambda x: x)

# returns category
print(df['col2'].dtype)
# returns object
print(other['col2'].dtype)

pandas.show_versions():

INSTALLED VERSIONS
------------------
commit: None
python: 3.4.3.final.0
python-bits: 64
OS: Linux
OS-release: 3.19.1-201.fc21.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: de_DE.utf8

pandas: 0.16.0
nose: 1.3.4
Cython: 0.21.2
numpy: 1.9.2
scipy: 0.15.1
statsmodels: 0.6.1
IPython: 3.0.0
sphinx: 1.2.3
patsy: 0.3.0
dateutil: 2.4.1
pytz: 2015.2
bottleneck: None
tables: 3.1.1
numexpr: 2.3.1
matplotlib: 1.4.3
openpyxl: 1.8.5
xlrd: 0.9.3
xlwt: None
xlsxwriter: 0.5.7
lxml: 3.4.0
bs4: 4.3.2
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None

[jreback]

@sebp well this is still an open issue; issues are closed if they are merged in
pull requests are welcome