python - Pandas:根据条件用另一个数据帧值替换数据帧中的值

Question

我有以下数据框:

df1:

+-----+------+------+------+------+------+
| No. | col1 | col2 | col3 | Type | ...  |
+-----+------+------+------+------+------+
| 123 |    2 |    5 |    2 | MN   | ...  |
| 453 |    4 |    3 |    1 | MN   | ...  |
| 146 |    7 |    9 |    4 | AA   | ...  |
| 175 |    2 |    4 |    3 | MN   | ...  |
| 643 |    0 |    0 |    0 | NAN  | ...  |
+-----+------+------+------+------+------+

df2:

+-----+------+------+------+------+
| No. | col1 | col2 | col3 | Type |
+-----+------+------+------+------+
| 123 |   24 |   57 |   22 | MN   |
| 453 |   41 |   39 |   15 | MN   |
| 175 |   21 |   43 |   37 | MN   |
+-----+------+------+------+------+

我想用 df2 中的相应值替换 df1 中的 col1、col2 和 col3 如果 Type 等于 MN

期望的输出:

df1:

+-----+------+------+------+------+-----+
| No. | col1 | col2 | col3 | Type | ... |
+-----+------+------+------+------+-----+
| 123 |   24 |   57 |   22 | MN   | ... |
| 453 |   41 |   39 |   15 | MN   | ... |
| 146 |    7 |    9 |    4 | AA   | ... |
| 175 |   21 |   43 |   37 | MN   | ... |
| 643 |    0 |    0 |    0 | NAN  | ... |
+-----+------+------+------+------+-----+

编辑

我试过:

df1[df1.Type == 'MN'] = df2.values

但是我得到这个错误:

ValueError: Must have equal len keys and value when setting with an ndarray

猜测原因是 df2 的列数不相等。那么我如何确保只有特定列 (col1 - col3) 在 df1 中被替换？

Answer 1

我认为需要combine_first按 No. 列匹配:

#filter only `MN` rows if necessary
df22 = df2[df2['Type'] == 'MN'].set_index('No.')
df1 = df22.combine_first(df1.set_index('No.')).reset_index().reindex(columns=df1.columns)
print (df1)

   No.  col1  col2  col3 Type  col
0  123  24.0  57.0  22.0   MN  ...
1  146   7.0   9.0   4.0   AA  ...
2  175  21.0  43.0  37.0   MN  ...
3  453  41.0  39.0  15.0   MN  ...
4  643   0.0   0.0   0.0  NAN  ...