Pandas: fill in NaN values with dictionary references another column

This can be done simply

df['B'] = df['B'].fillna(df['A'].apply(lambda x: dict.get(x)))

This can work effectively for a bigger dataset as well.

You can map dict values inside fillna

df.B = df.B.fillna(df.A.map(dict))

print(df)

    A   B
0   a   2
1   b   5
2   c   4