Pandas如何用重复的键替换行中的NaN

import pandas as pd
import numpy as np

data = {
    'key': ['A', 'B', 'A', 'C'],
    'value1': [1, 2, np.nan, 4],
    'value2': [np.nan, 6, 7, 8]
}

df = pd.DataFrame(data)
print(df)

  key  value1  value2
0   A     1.0     NaN
1   B     2.0     6.0
2   A     NaN     7.0
3   C     4.0     8.0

df['value1'] = df.groupby('key')['value1'].transform('first')
df['value2'] = df.groupby('key')['value2'].transform('first')
print(df)

  key  value1  value2
0   A     1.0     7.0
1   B     2.0     6.0
2   A     1.0     7.0
3   C     4.0     8.0