Python で上書きせずに既存の Excel シートに Pandas DataFrame を追加するにはどうすればよいですか?

import pandas as pd

df = pd.DataFrame(...)

with pd.ExcelWriter('existing_sheet.xlsx', mode='a', engine='openpyxl') as writer:
    df.to_excel(writer, sheet_name='Sheet1', header=False, index=False, startrow=writer.sheets['Sheet1'].max_row + 1)

import pandas as pd
from openpyxl import load_workbook

df = pd.DataFrame(...)

# Load existing Excel workbook
book = load_workbook('existing_sheet.xlsx')
sheet = book['Sheet1']

# Find last row in existing Excel sheet
last_row = sheet.max_row

# Append DataFrame data to existing sheet starting from last row + 1
pd.DataFrame.to_excel(df, book, sheet_name='Sheet1', startrow=last_row + 1, index=False)

# Save changes to existing Excel file
book.save('existing_sheet.xlsx')