BUG: Create empty dataframe with string dtype fails "data type not understood"

[selik]

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• (optional) I have confirmed this bug exists on the master branch of pandas.

---

Code Sample, a copy-pastable example

In [1]: import pandas as pd

In [2]: pd.DataFrame({'a': ['b']}, dtype='string')
Out[2]:
   a
0  b

In [3]: pd.DataFrame(columns=['a'], dtype='string')
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-2e577cc30885> in <module>
----> 1 pd.DataFrame(columns=['a'], dtype='string')

~/lib/miniconda/envs/xport/lib/python3.8/site-packages/pandas/core/frame.py in __init__(self, data, index, columns, dtype, copy)
    433             )
    434         elif isinstance(data, dict):
--> 435             mgr = init_dict(data, index, columns, dtype=dtype)
    436         elif isinstance(data, ma.MaskedArray):
    437             import numpy.ma.mrecords as mrecords

~/lib/miniconda/envs/xport/lib/python3.8/site-packages/pandas/core/internals/construction.py in init_dict(data, index, columns, dtype)
    232         # no obvious "empty" int column
    233         if missing.any() and not is_integer_dtype(dtype):
--> 234             if dtype is None or np.issubdtype(dtype, np.flexible):
    235                 # GH#1783
    236                 nan_dtype = object

~/lib/miniconda/envs/xport/lib/python3.8/site-packages/numpy/core/numerictypes.py in issubdtype(arg1, arg2)
    391     """
    392     if not issubclass_(arg1, generic):
--> 393         arg1 = dtype(arg1).type
    394     if not issubclass_(arg2, generic):
    395         arg2_orig = arg2

TypeError: data type not understood

In [4]: pd.DataFrame(columns=['a'], dtype='float')
Out[4]:
Empty DataFrame
Columns: [a]
Index: []

Problem description

When trying to create an empty dataframe, one can specify some dtypes, like 'float', but dtype='string' raises TypeError: data type not understood.

Expected Output

Output of pd.show_versions()

INSTALLED VERSIONS

commit : None
python : 3.8.2.final.0
python-bits : 64
OS : Darwin
OS-release : 19.4.0
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.0.3
numpy : 1.18.2
pytz : 2019.3
dateutil : 2.8.1
pip : 20.0.2
setuptools : 46.1.3.post20200330
Cython : None
pytest : 5.4.1
hypothesis : None
sphinx : 3.0.1
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 2.11.2
IPython : 7.13.0
pandas_datareader: None
bs4 : None
bottleneck : None
fastparquet : None
gcsfs : None
lxml.etree : None
matplotlib : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pytables : None
pytest : 5.4.1
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
xlsxwriter : None
numba : None

[jorisvandenbossche]

@selik thanks for the report!

[simonjayhawkins]

xref #27953