Open existing workbook with ExcelWriter

Question

I have a code below that creates a workbook, then uses pandas ExcelWriter to open it and create a another worksheet called '1'. But it seems like ExcelWriter will overwrite the entire workbook. How do I open an existing excel workbook with ExcelWriter?

import pandas as pd

df=pd.DataFrame({'a':[1,2],'b':[3,4]})

df.to_excel('test.xlsx')

writer=pd.ExcelWriter('test.xlsx',engine='xlsxwriter')
df.to_excel(writer,sheet_name='1')

writer.close()