title: Python to excel, openpyxl and file format not valid
body: <p>The following is a simple snippet to open a .xlsm file, write a few values to it with python, and save it. </p>
  
  <pre><code>import openpyxl
  from openpyxl import load_workbook
  
  def toExcel():
      wb = load_workbook(filename="C:\\Users\\Mark\\Documents\\Test.xlsm")
      ws = wb.worksheets[0]
      ws.cell(row=1, column=1).value = 'foo'
      ws['A2'] = 'bar'
      wb.save("C:\\Users\\Mark\\Documents\\Test1.xlsm")
  
  toExcel()
  </code></pre>
  
  <p>While the file opens and saves, it mentions file format not valid / corrupt and cannot open. If the .xlsm is removed from the wb.save, it will save and open after selecting excel with Open With. Why is the file format not valid as is?</p>