Pandas reindex(fill_value=None) fills with np.NaN instead of None

Code Sample, a copy-pastable example if possible

In [12]: import pandas as pd

In [13]: s = pd.Series(['a', 'b'])

In [14]: s.reindex([0,1,2], fill_value=None)
Out[14]: 
0      a
1      b
2    NaN
dtype: object

Expected Output

0      a
1      b
2    None
dtype: object

output of pd.show_versions()

commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-34-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.0
nose: None
pip: 8.1.2
setuptools: 25.2.0
Cython: 0.20.1
numpy: 1.8.1
scipy: 0.17.0
statsmodels: 0.6.1
xarray: None
IPython: 1.1.0
sphinx: 1.2b1
patsy: 0.4.1
dateutil: 2.4.2
pytz: 2013b
blosc: None
bottleneck: None
tables: 3.1.1
numexpr: 2.4
matplotlib: None
openpyxl: 2.3.0
xlrd: 0.9.3
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 0.9.8
pymysql: None
psycopg2: None
jinja2: 2.7
boto: None

in source

This is happening in BlockManager.reindex_indexer() https://github.com/pydata/pandas/blob/8af626474f6f314527a9ad3f15403aa2dd8c402d/pandas/core/internals.py#L3820-L3822

Comment From: jreback

this is by definition np.nan is the missing indicator

Comment From: jreback

http://pandas.pydata.org/pandas-docs/stable/text.html

Comment From: nekobon

I think it's confusing to get NaN when we give fill_value=None explicitly, without warnings or exceptions.

According to reindex's document http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.reindex.html:

fill_value : scalar, default np.NaN
    Value to use for missing values. Defaults to NaN, but can be any “compatible” value

Perhaps we can improve the document to mention None is not a compatible value there.

Comment From: jreback

the default argument is None, meaning it's not passed

this is fairly standard convention

what I would take for documentation is a small section in. missing.rst to add that strings use np.nan as the missing value

near the top

Comment From: nekobon

I see that not supporting None here is consistent with this issue on fillna.

It's true that None by default is standard in python, but it's also common to use a sentinel object (object()) when None could be meaningful. This would let us use None on both fillna and fill_value, and I think it's an improvement. What do you think?

Comment From: jreback

you could use s sentinel but we don't allow None filling for a variety of reasons

Comment From: nekobon

Could you share with me some of those reasons? I've been using None in Series and DataFrames, so I'm curious about why we shouldn't.

Comment From: jreback

http://pandas.pydata.org/pandas-docs/stable/missing_data.html

Comment From: nekobon

Thank you