如何让一些pandas列值默认为同一行不同列中的另一个值？

df['目标列'] = df['目标列'].fillna(df['另一列'], method='ffill')

import pandas as pd

# 创建示例数据
data = {'A': [1, 2, None, 4, None],
        'B': [None, 5, 6, None, 8],
        'C': [9, None, 11, 12, 13]}
df = pd.DataFrame(data)

# 将列A的缺失值填充为列B的值
df['A'] = df['A'].fillna(df['B'], method='ffill')

# 将列B的缺失值填充为列C的值
df['B'] = df['B'].fillna(df['C'], method='ffill')

print(df)

     A    B     C
0  1.0  9.0   9.0
1  2.0  5.0   5.0
2  2.0  6.0  11.0
3  4.0  6.0  12.0
4  4.0  8.0  13.0