在None中找不到引用'set_index'

data_df = pd.read_csv(r'C:\Users\admin\PycharmProjects\pythonProject\data2.csv')
data_df.head()
data_df.isnull().any(axis=1)
data_df = data_df.fillna(0)
data_df.isnull().any(axis=1)
data_df.head()
df = data_df.sort_values(by='成交量', inplace=True, ascending=False)
data_df.head(10)
print(data_df.head(10))

import pandas as pd

df = df.set_index('股票名称')
df.to_csv("data1.csv", encoding='utf-8')
data_df = pd.read_csv("data1.csv")

这个问题如何解决各位大佬
AttributeError: 'NoneType' object has no attribute 'set_index'

df为None时候特殊处理下呗