从另一个数据框填充一个数据框的列

问题描述

我正在尝试根据条件从另一个数据框填充一个数据框的列.假设我的第一个数据帧是df1，第二个数据帧是df2.

I'm trying to fill a column of a dataframe from another dataframe based on conditions. Let's say my first dataframe is df1 and the second is named df2.

# df1 is described as bellow :
+------+------+
| Col1 | Col2 |
+------+------+
|   A  |  1   |
|   B  |  2   |
|   C  |  3   |
|   A  |  1   |
+------+------+

还有

# df2 is described as bellow :
+------+------+
| Col1 | Col2 |
+------+------+
|   A  |  NaN |
|   B  |  NaN |
|   D  |  NaN |
+------+------+

Col1的每个不同值都有她的ID号(在Col2中)，所以我要在df2.Col2中填充NaN值，其中df2.Col1 == df1.Col1.这样我的第二个数据帧看起来就像:

Each distinct value of Col1 has her an id number (In Col2), so what I want is to fill the NaN values in df2.Col2 where df2.Col1==df1.Col1 .So that my second dataframe will look like :

# df2 :
+------+------+
| Col1 | Col2 |
+------+------+
|   A  |  1   |
|   B  |  2   |
|   D  |  NaN |
+------+------+

我正在使用Python 2.7

I'm using Python 2.7

推荐答案

使用 drop_duplicates 与 set_index 和 combine_first :

df = df2.set_index('Col1').combine_first(df1.drop_duplicates().set_index('Col1')).reset_index()

如果只需要在 id 列中检查重复项:

If need check dupes only in id column:

df = df2.set_index('Col1').combine_first(df1.drop_duplicates().set_index('Col1')).reset_index()

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