df1 = pd.DataFrame.from_records([('2015-07-09 12:00:00', 1, 1, 1), (
    '2015-07-09 13:00:00', 1, 1, 1), ('2015-07-09 14:00:00', 1, 1, 1), (
    '2015-07-09 15:00:00', 1, 1, 1)], columns=['Dt', 'A', 'B', 'C']).set_index(
    'Dt')
Setting DataFrame values with enlargement