pandas该如何批量用一行的平均值来填充这一行的缺失值

import pandas as pd
import numpy as np

df = pd.DataFrame(
    {"a": [1, 2, 3, np.nan],
     "b": [4, np.nan, 5, 6],
     "c": [7, 8, np.nan, 9]},
)
print(df)
columns = list(df.columns)

def my_function(row):
    """这个参数是每行的row"""
    avg = row.mean()
    for column in columns:
        if np.isnan(row[column]):
            row[column] = avg
    return row

df[columns] = df.apply(my_function, axis=1, result_type='expand')
print(df)