연습용 데이터프레임 만들기

>>> import pandas as pd
>>> data = {'col1': [1, 2, 3, 4], 'col2': [5, 6, 7, 8], 'col3': [9, 10, 11, 12]}
>>> df = pd.DataFrame(data)
>>> df
   col1  col2  col3
0     1     5     9
1     2     6    10
2     3     7    11
3     4     8    12