python - 仅在同一行索引内应用函数？

Question

我有一个包含 2 个排序索引的数据框，我想按 col2 排序的顺序仅在 col1 内的列上应用 diff .

mini_df = pd.DataFrame({'col1': ['A', 'B', 'C', 'A'], 'col2': [1,2,3,4],  'col3': [1,4,7,3]})
mini_df = mini_df.set_index(['col1', 'col2']).sort_index()
mini_df['diff'] = mini_df.col3.diff(1)

这给了我

              col3    diff
col1    col2  
__________________________
A        1      1      nan
         4      3       2
B        2      4       1
C        3      7       3

上面它按行应用diff。我想要的是

              col3    diff
col1    col2  
__________________________
A        1      1      nan
         4      3       2
B        2      4      nan
C        3      7      nan

Answer 1

您需要使用 groupby 将 diff 应用于每个组:

mini_df = pd.DataFrame({'col1': ['A', 'B', 'C', 'A'], 'col2': [1,2,3,4],  'col3': [1,4,7,3]})
mini_df = mini_df.set_index(['col1', 'col2']).sort_index()

mini_df['diff'] = mini_df.groupby(axis=0, level='col1')['col3'].diff()