ENH: add option to get nullable dtypes to pd.read_csv

[galipremsagar]

• 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.

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Code Sample, a copy-pastable example

In[2]: import pandas as pd
In[3]: s = pd.Series([1, 2, None, 3], dtype=pd.UInt64Dtype())
In[4]: s
Out[4]: 
0       1
1       2
2    <NA>
3       3
dtype: UInt64
In[5]: df = pd.DataFrame({"a":s})
In[6]: df
Out[6]: 
      a
0     1
1     2
2  <NA>
3     3
In[7]: df.to_csv('temp.csv')
In[8]: pd.read_csv('temp.csv')
Out[8]: 
   Unnamed: 0    a
0           0  1.0
1           1  2.0
2           2  NaN
3           3  3.0

Problem description

While reading csv, pandas should likely read in the above dataframe column as a nullable integer dtype instead of converting to floating data type.

Expected Output

pd.read_csv('temp.csv')
Out[8]: 
   Unnamed: 0  a
0           0  1
1           1  2
2           2  <NA>
3           3  3

Output of pd.show_versions()

INSTALLED VERSIONS

commit : 2a7d332
python : 3.7.8.final.0
python-bits : 64
OS : Linux
OS-release : 5.4.0-48-generic
Version : #52-Ubuntu SMP Thu Sep 10 10:58:49 UTC 2020
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8
pandas : 1.1.2
numpy : 1.19.1
pytz : 2020.1
dateutil : 2.8.1
pip : 20.2.3
setuptools : 49.6.0.post20200917
Cython : 0.29.21
pytest : 6.0.2
hypothesis : 5.28.0
sphinx : 3.2.1
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 2.11.2
IPython : 7.18.1
pandas_datareader: None
bs4 : None
bottleneck : None
fsspec : 0.8.2
fastparquet : None
gcsfs : None
matplotlib : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 1.0.1
pytables : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
numba : 0.51.2

[jreback]

there is an issue about this already - pls search

this is a major breaking change and cannot be implemented as default yet

[jorisvandenbossche]

@galipremsagar Thanks for opening the issue. As Jeff mentions, as long as the nullable dtypes are not yet the default dtypes, we can't read a csv using those dtypes by default (and a CSV file does not hold any type information that it was created from nullable dtypes).

However, the idea is that in the meantime, we can provide a keyword to opt-in to use nullable dtypes when doing IO, see #29752 for a general issue about this (and #31242 for a specific PR adding this to read_parquet). So we can certainly add such a keyword to CSV reading as well.