How to proceed with `None` value in pandas fillna

Setup
Consider the sample dataframe df

df = pd.DataFrame(dict(A=[1, None], B=[None, 2], C=[None, 'D']))

df

     A    B     C
0  1.0  NaN  None
1  NaN  2.0     D

I can confirm the error

df.fillna(dict(A=1, B=None, C=4))

ValueError: must specify a fill method or value

This happens because pandas is cycling through keys in the dictionary and executing a fillna for each relevant column. If you look at the signature of the pd.Series.fillna method

Series.fillna(value=None, method=None, axis=None, inplace=False, limit=None, downcast=None, **kwargs)

You'll see the default value is None. So we can replicate this error with

df.A.fillna(None)

Or equivalently

df.A.fillna()

I'll add that I'm not terribly surprised considering that you are attempting to fill a null value with a null value.

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What you need is a work around

Solution
Use pd.DataFrame.fillna over columns that you want to fill with non-null values. Then follow that up with a pd.DataFrame.replace on the specific columns you want to swap one null value with another.

df.fillna(dict(A=1, C=2)).replace(dict(B={np.nan: None}))

     A     B  C
0  1.0  None  2
1  1.0     2  D

In case you want to normalize all of the nulls with python's None.

df.fillna(np.nan).replace([np.nan], [None])

The first fillna will replace all of (None, NAT, np.nan, etc) with Numpy's NaN, then replace Numpy's NaN with python's None.