openpyxl获取工作表报错TypeError: 'list' object is not callable

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>>> import openpyxl
>>> wb = openpyxl.load_workbook('example.xlsx')
>>> wb.sheetnames()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'list' object is not callable

新打开python，输入上述代码，报错list类不能使用，但是也没有变量占用list标识符啊