python dataframe NaN处理方式

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
# 将NaN替换为None
print data.where(data.notnull(), None)

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