Pandas 基于连接将列从一个数据帧添加到另一个数据帧

Pandas add column from one dataframe to another based on a join

提问by robert

Assume I have 2 dataframes. I want to add a column of dataframe 1 to dataframe 2 based on a column lookup. If the join is not possible, I want in the extra column a certain constant (so I can filter for that).

假设我有 2 个数据框。我想根据列查找将一列数据框 1 添加到数据框 2。如果连接是不可能的，我希望在额外的列中有一个特定的常量（所以我可以过滤它）。

Graphically:

图形化：

Code:

代码：

import pandas as pd
import numpy as np

data = np.array([['','Col1','Col2'],
                ['Row1','2','TWO'],
                ['Row2','1','ONE']]
            )

data2 = np.array([['','Col3','Col4'],
                ['Row1','1','T1'],
                ['Row2','2','T2'],
                ['Row3','3','T3']]
            )

df = pd.DataFrame(data=data[1:,1:],
                  index=data[1:,0],
                  columns=data[0,1:])

df2 = pd.DataFrame(data=data2[1:,1:],
                  index=data2[1:,0],
                  columns=data2[0,1:])

result_df = df2 + join Col2 based on df2.Col3 = df.Col1. Add certain string constant if join fails. 

print(df)
print(df2)
print(result_df)

回答by jezrael

Use joinor map:

使用join或map：

df = df2.join(df.set_index('Col1'), on='Col3')
print (df)
     Col3 Col4 Col2
Row1    1   T1  ONE
Row2    2   T2  TWO
Row3    3   T3  NaN

---

---

df2['Col2'] = df2['Col3'].map(df.set_index('Col1')['Col2'])
print (df2)
     Col3 Col4 Col2
Row1    1   T1  ONE
Row2    2   T2  TWO
Row3    3   T3  NaN