Code Sample

>>> import pandas as pd
>>> import numpy as np
>>> pd.Series(["foo",np.nan]).astype(str)

Output

0 foo
1 nan # nan string
dtype: object

Expected output

0 foo
1 NaN # np.nan
dtype: object

Problem description

Upon converting this Series I would expect np.nan to remain np.nan but instead it is casted to string "nan". Maybe I'm alone in this and you'd actually expect that (I can't see a realistic use case for these string "nan" but well...).
So I could figure out than upon using the code sample the Series' values are processed through astype_unicode in pandas._libs.lib.

There is a skipna argument in astype_unicode and I thought it would get passed along when using pd.Series.astype(str,skipna = True) but it does not. The docstring of pd.Series.astype does not mention skipna explicitely but mentions kwargs so I tried doing this while printing skipna in astype_unicode:

pd.Series(["foo",np.nan]).astype(str, skipna = True)

Skipna stayed to the default value False in astype_unicode so it does not get passed along.

However when using astype_unicode directly setting skipna to True will not change the output of the the code sample anyways because checknull does not seem to work properly.
You can test that by printing the result of checknull in lib.pyx as I did here:
https://github.com/ThibTrip/pandas/commit/4a5c8397304e3026456d864fd5aeb7b8b9adca5f

Input

>>> from pandas._lib.lib import astype_unicode
>>> import numpy as np
>>> astype_unicode(np.array(["foo",np.nan]))

Output

foo is null? False
nan is null? False
array(['foo', 'nan'], dtype=object)

Expected output

foo is null? False
nan is null? True
array(['foo', NaN], dtype=object)

I tried patching it (so not a proper fix) using the code below but it din't work.
Also would be nice to be able to do pd.Series.astype(str,skipna = True). Whether skipna should then be True or False as default is another matter.

>>> if not (skipna and checknull(arr_i)):
>>>     if arr_i is not np.nan:
>>>         arr_i = unicode(arr_i)

All of this was done in a developper version I installed today (see details below). The only alteration is the code in my commit linked above.
Sorry if this has been referenced before I searched in various ways and could not find anything except a similar issue with pd.read_excel and dtype str:

nikoskaragiannakis@694849d

Also very sorry for the mess with the commits I got a bit confused during my investigation (also I did not get enough sleep). Is it possible to delete all but my last commit? The other ones are irrelevant.

Cheers Thibault