import pandas as pd
import numpy as np    
data = {'a' : 0., 'b' : 1., 'c' : 2.}
s = pd.Series(data,index=['b','c','d','a'])
print (s)
OUTPUT IS : 
b    1.0
c    2.0
d    NaN
a    0.0
dtype: float64
I want to assign some values to NaN. Finally i got the answer. I was thinking to remove the nan with empty string "" and below is the code.
        data = pd.read_excel(i)
        data1 = data.replace(np.nan, '', regex=True)
        data1.columns = data1.columns.str.replace('Unnamed.*', '')
