熊貓根據連接將列從一個數據框添加到另一個數據框

Question

假設我有2個數據幀。我想基於列查找將數據幀1列添加到數據幀2。如果連接不可行，我想在額外的列中保持一定的常量（所以我可以過濾）。

圖形：

代碼：

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) 

Answer 1

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