info = pd.DataFrame({'names': ['joe', 'mark', 'tim', 'frank', 'joe', 'bill'
    ], 'classification': ['thief', 'thief', 'good', 'thief', 'good', 'thief']})
what = pd.merge(names, info, how='outer')
what.fillna('unknown', inplace=True)
pandas merge dataframe fill in missing values