Regression: None in string input is wrongly cast to the "None" string

[pitrou]

In Pandas 0.22.0:

>>> df = pd.DataFrame({'data': ['x', None]}, dtype=str)
>>> df['data'].tolist()
['x', None]

In Pandas 0.23.0:

>>> df = pd.DataFrame({'data': ['x', None]}, dtype=str)
>>> df['data'].tolist()
['x', 'None']

>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-20-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: fr_FR.UTF-8
LOCALE: fr_FR.UTF-8

pandas: 0.23.0
pytest: 3.3.2
pip: 10.0.1
setuptools: 38.4.0
Cython: 0.28.2
numpy: 1.14.2
scipy: None
pyarrow: 0.9.1.dev14+g6599ab0.d20180323
xarray: None
IPython: 6.2.1
sphinx: 1.6.7
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.1.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

[TomAugspurger]

This sounds vaguely familiar... Interestingly, these two are different.

In [6]: pd.Series(['a', None]).values.tolist()
Out[6]: ['a', None]

In [7]: pd.Series(['a', None], dtype='str').values.tolist()
Out[7]: ['a', 'None']

[pitrou]

Hmm, the "str" dtype blindly represents everything:

>>> pd.Series(['a', None, 0.4j, object()], dtype='str').values.tolist()
['a', 'None', '0.4j', '<object object at 0x7fc22cf8e6d0>']

On Pandas 0.22.0, however, the "str" dtype specification seems completely ignored:

>>> pd.Series(['a', None, 0.4j, object()], dtype='str').values.tolist()
['a', None, 0.4j, <object at 0x7fe5ddcd1e90>]

[TomAugspurger]

I think we're hitting the else in

pandas/pandas/core/series.py

Line 4044 in 501f041

if is_object_dtype(dtype) and (is_list_like(subarr) and

, when we want to go down the if.

(Pdb) dtype
dtype('<U')
(Pdb) is_object_type(dtype)
*** NameError: name 'is_object_type' is not defined
(Pdb) dtype
dtype('<U')
(Pdb) is_object_dtype(dtype)