pd.fillna replacement value not appearing in correct position in csv file

Question

I am very new to python and trying to fill NaN values in a very large csv file with a space using pd.fillna but the spaces all appear at the end of each row with NaN values. Code is listed below.

reader = pd.read_csv(path, sep='\s+', skipinitialspace=True, dtype=str, engine='python', names=headers)
spacedReader = reader.fillna(value=' ', axis=1)
spacedReader.to_csv(path, sep=',', quoting=csv.QUOTE_NONE, escapechar=' ', header=headers)

001  alb  272  nc      
002  NaN  NaN  nc
NaN  alb  768  nc

will produce...

001,alb,272,nc
002,nc, , 
alb,768,nc, 

And I need...

001,alb,272,nc
002, , , nc
 ,alb,768,nc

I would greatly appreciate any help!

Answer 1

You can try something like this

import numpy as np
import pandas as pd

df = pd.DataFrame([[np.nan, 2, np.nan, 0],
                   [3, 4, np.nan, 1],
                   [np.nan, np.nan, np.nan, 5],
                   [np.nan, 3, np.nan, 4]],
                  columns=list('ABCD'))


df1 = df.replace(np.nan, '', regex=True)

df1.head()

output

    A   B   C   D
0       2       0
1   3   4       1
2               5
3       3       4