title: How to append a dataframe to existing excel sheet?
body: <p>The function gets called several times. I have kept a count so that if its called for the first time, A workbook is created. Then I write to that workbook using <code>pd.ExcelWrite()</code>. Next time <code>else:</code> gets executed and the same workbook is opened. Its first sheet is selected, its last row is found. And DataFrame is written on that row.
  This is my code:</p>
  
  <pre><code>def WriteFile (df):
    if count1 == 1:
        workbook = xlsxwriter.Workbook('pandas_simple.xlsx')
        writer = pd.ExcelWriter('pandas_simple.xlsx', engine='xlsxwriter')
        df.to_excel(writer, index=False )
        workbook.close()
    else:
        book = open_workbook('pandas_simple.xlsx')
        sheet_names = book.sheet_names()
        sheet = book.sheet_by_name(sheet_names[0])
        row = sheet.nrows
        writer = pd.ExcelWriter('pandas_simple.xlsx', engine='xlsxwriter')
        df.to_excel(writer, index=False, header=False, startrow = row )
  </code></pre>
  
  <p>I get this exception:</p>
  
  <pre><code>Exception Exception: Exception('Exception caught in workbook destructor. Explicit close() 
  may be required for workbook.',) in &lt;bound method Workbook.__del__ of &lt;xlsxwriter.workbook.Workbook 
  object at 0x000000000A143860&gt;&gt; ignored Exception
  </code></pre>
  
  <p>And my pandas_simple.xlsx is also empty after execution of code. What am I doing wrong? </p>