將dataframe中的NaN替換成希望的值

import pandas as pd
df1 = pd.DataFrame([{'col1':'a', 'col2':1}, {'col1':'b', 'col2':2}])
df2 = pd.DataFrame([{'col1':'a', 'col3':11}, {'col1':'c', 'col3':33}])

data = pd.merge(left=df1, right=df2, how='left', left_on='col1', right_on='col1')
print data
# 將NaN替換為None
print data.where(data.notnull(), None
 
)

輸出結果:

  col1  col2  col3
0    a     1    11
1    b     2   NaN
  col1  col2  col3
0    a     1    11
1    b     2  None