BUG: replacing a string with None causes an unexpected forward fill

[PedroGFonseca]

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

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Note: Please read this guide detailing how to provide the necessary information for us to reproduce your bug.

Code Sample, a copy-pastable example

# Your code here
df = pd.DataFrame(np.zeros((3, 3)))
df.iloc[2, 2] = ''
df = df.replace('', None)
print(df.iloc[2, 2] == '') # prints False
print(df.iloc[2, 2] == df.iloc[1, 2]) # prints True

Problem description

[this should explain why the current behaviour is a problem and why the expected output is a better solution]

Expected Output

Replacing a string with a None should introduce a None, not forward fill the previous result.

Output of pd.show_versions()

INSTALLED VERSIONS

commit : db08276
python : 3.6.5.final.0
python-bits : 64
OS : Darwin
OS-release : 19.6.0
Version : Darwin Kernel Version 19.6.0: Thu Jun 18 20:49:00 PDT 2020; root:xnu-6153.141.1~1/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.1.3
numpy : 1.19.2
pytz : 2018.9
dateutil : 2.8.1
pip : 20.2
setuptools : 49.2.0
Cython : None
pytest : 5.4.3
hypothesis : None
sphinx : 1.8.3
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.5.1
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 2.10.1
IPython : 7.13.0
pandas_datareader: None
bs4 : 4.9.0
bottleneck : None
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : 3.2.2
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pytables : None
pyxlsb : None
s3fs : None
scipy : 1.4.1
sqlalchemy : 1.3.16
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
numba : None

[PedroGFonseca]

Note: replacing with np.nan does not cause this problem.

[asishm]

this is documented behavior.

to replace with None, you need to do df.replace({'': None}) per the docs

perhaps making it a bit more clearer in the docs might help?

[PedroGFonseca]

Thanks for the fast answer. I would suggest that it may cause some silent errors, given that it does not fail.

The behavior does indeed make sense, and it was 100% my mistake, but I fear that having "pad" as the default method may result in unexpected behaviors. Would it not make sense for it to not have a default method, and make the user make an explicit decision?

[asishm]

I would agree that the behavior is confusing. Not sure what the rationale was for this specific scenario.

A behavior change would probably need to be discussed (and then maybe go through a deprecation cycle if there is consensus).

[Jimsparkle]

Just come here to mention that this is very confusing