把xlsx变成CSV

import pandas as pd
import numpy as np
import matplotlib.pyplot as plt
#df.to_excel('C:/Users/Asus/Desktop/1.xlsx',sheet_name='dfg')
df=pd.read_excel('D:/Python文件/Python成绩.xlsx',index_col=None,na_values=['NA'])
print(df)
for i in range(len(df.index)):
   # print(df.iloc[i,1])
    for j in range(1,len(df.columns)):
        if df.iloc[i,j]=='优秀':
            df.iat[i,j]=90
        elif df.iloc[i,j]=='良好':
            df.iat[i,j]=80
        elif df.iloc[i,j]=='合格':
            df.iat[i,j]=60
        else:
            df.iat[i,j]=1
df.to_csv('D:/Python文件/Python成绩.xlsx.csv')
print(df)