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import pandas as pd
import openpyxl
workbook = openpyxl.load_workbook("test.xlsx")
writer = pd.ExcelWriter('test.xlsx', engine='openpyxl')
writer.book = workbook
writer.sheets = dict((ws.title, ws) for ws in workbook.worksheets)
data_df.to_excel(writer, 'Existing_sheetname')
writer.save()
writer.close()