Error:cannot convert float NaN to integer in pandas

Pandas doesn’t have the ability to store NaN values for integers. Strictly speaking, you could have a column with mixed data types, but this can be computationally inefficient. So if you insist, you can do

df['a'] = df['a'].astype('O')
df.loc[df['a'].notnull(), 'a'] = df.loc[df['a'].notnull(), 'a'].astype(int)