Python Update two dataframes with identical columns and a few differing rows-CodePudding

I am joining two data frames that have the same columns. I wanted to update the first dataframe. However, the my code creates additional columns but it is not updating.

My code:

left = pd.DataFrame({"key": ["K0", "K1", "K2", "K3"],
            "A": ["NaN", "NaN", "NaN", "NaN"],
        "B": ["B0", "B1", "B2", "B3"],})
 right = pd.DataFrame({"key": ["K1", "K2", "K3"],    
        "A": ["C1", "C2", "C3"],    
        "B": [ "B1", "B2", "B3"]})
result = pd.merge(left, right, on="key",how='left')

Present output:

result = 
  key  A_x B_x  A_y  B_y
0  K0  NaN  B0  NaN  NaN
1  K1  NaN  B1   C1   B1
2  K2  NaN  B2   C2   B2
3  K3  NaN  B3   C3   B3

Expected output:

result = 
  key  B    A  
0  K0  B0  NaN 
1  K1  B1   C1 
2  K2  B2   C2 
3  K3  B3   C3

CodePudding user response：

Use combine_first:

result = left.set_index("key").combine_first(right.set_index("key")).reset_index()
print(result)

Output

  key    A   B
0  K0  NaN  B0
1  K1   C1  B1
2  K2   C2  B2
3  K3   C3  B3