df = pd.DataFrame({'from_to': ['ShangHAI_BeiJing', 'ShangHAI_Chongqing', 'BeiJing_chengdu',
                               'tianJin_PaRis', 'Beijing_shenzhen'],
                   'FlightNumber': [10001, 10011, np.nan,10021, 10035],
                    })
df['FlightNumber'] = df['FlightNumber'].interpolate().astype(int)
df
temp = df.from_to.str.split('_', expand=True)
temp.columns = ['from', 'to']
df = df.drop('from_to', axis=1)
df = df.join(temp)
df