将数据框附加到另一个数据框的列和Python常量

Question

Let's take these two dataframes :

df1 = pd.DataFrame([[1, 2], [3, 4]], columns=list('AB'))
df1
   A  B
0  1  2
1  3  4

df2 = pd.DataFrame([[5, 6], [7, 8]], columns=list('CD'))
df2
   C  D
0  5  6
1  7  8

I would like to add column C of df2 to column A of df1, and to put 9 in column B. To sum up, I would like to have :

df1
df1
   A  B
0  1  2
1  3  4
2  5  9
3  7  9

I tried numerous things with the append function but didn't succeed to find the right code. Could you please help me ?

Answer 1

df1.append(df2.rename(columns={'C':'A'}).drop(columns='D'), ignore_index=True) \
   .fillna(9).astype(int)

   A  B
0  1  2
1  3  4
2  5  9
3  7  9

Answer 2

Another alternative based on @splash58's answer :

df1.append(df2.rename(columns={'C':'A'}).drop(df2.columns.difference(['C']), 1), ignore_index=True,sort=False) \
   .fillna(9).astype(int)