如何在numpy中将pandas数据帧中的字符串转换为三维数组

import numpy as np
import pandas as pd

df = pd.DataFrame({'A': ['a', 'b', 'c'], 'B': ['d', 'e', 'f'], 'C': ['g', 'h', 'i']})

def str_to_array(s):
    return np.array(list(s)).reshape(3, 3)
    
str_to_array = np.vectorize(str_to_array)

array_3d = str_to_array(df.values.flatten())