title: pandas writing to excel sheet deleting other sheets in file
body: <p>I have simple code to export python dataframe to existing excel file with sheets
  but the writer keep deleting the existing sheet from the file </p>
  
  <pre><code>read = pd.ExcelFile('Saw_Load.xlsx')
  print(read.sheet_names)
  writer = pd.ExcelWriter('Saw_Load.xlsx')
  result.to_excel(writer,'saw', index = False)
  read2 = pd.ExcelFile('Saw_Load.xlsx')
  print(read2.sheet_names)
  writer.save()
  </code></pre>
  
  <p>Here is the output i am getting</p>
  
  <pre><code>['saw', 'Pivot']
  ['saw']
  </code></pre>
  
  <p>We can clearly see before to_excel function was used there were 2 sheets (saw,Pivot).  After there  is only one 'saw'</p>
  
  <p>It could be a simple fix in formula but couldn't seem to find anything that works.  Any help will be appreciated </p>
  
  <p>Thanks</p>