Python How to use ExcelWriter to write into an existing worksheet

Use load_book from openpyxl – see xlsxwriter and openpyxl docs:

import pandas as pd
from openpyxl import load_workbook

book = load_workbook('test.xlsx')
writer = pd.ExcelWriter('test.xlsx', engine="openpyxl") 
writer.book = book
writer.sheets = dict((ws.title, ws) for ws in book.worksheets)

df.to_excel(writer, sheet_name="tab_name", other_params)

writer.save()