python中产生一个矩阵，元素相同且为字符串，另外用 pd.DataFrame构造矩阵

import numpy as np
import pandas as pd
a=np.full((3, 1), 'no exist')
df = pd.DataFrame([['A', 2], ['A', 4], ['B', 6]])
print(a)
print(df)
#df[1]=df[1].astype(str)
df[1]=a#第二列赋值
print(df)

height = 3
width = 5
df_0 = pd.DataFrame(0, index=range(height), columns=range(width))
print(df_0)
#df_0[1]=df_0[1].astype(str)
df_0[1]=a
print(df_0)

b=df_0.at[0,0]
print(b)

df_0.iloc[2,2] = 1111.67890
print(df_0)

df_0= round(df_0,2)#两位小数显示
print(df_0)