How fill NA with respect of another column?

I want to fill NaN values in this way: put the same values as the column B if they have the same value in B


A    B
nan 'ra'
9   'ra'
5   'pa'

So NaN value in column A should be 9 because they have the same values in column B.

>Solution :

df['A'] = df.groupby('B')['A'].ffill().bfill()


>>> df
     A   B
0  9.0  ra
1  9.0  ra
2  5.0  pa

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