title: Writing data into Excel-Sheet using openpyxl isn't working
body: <p>Using <code>openpyxl</code>, I am trying to read data from an Excel-Workbook and write data to this same Excel-Workbook. Getting data from the Excel-Workbook works fine, but writing data into the Excel-Workbook does not work. With the code below I get the value from Cell <code>A1</code> in <code>Sheet1</code> and print it. Then I try to put some values into the cells <code>A2</code> and <code>A3</code>. This does not work.  </p>
  
  <pre><code>from openpyxl import Workbook
  from openpyxl import load_workbook
  
  
  wb = load_workbook("testexcel.xlsm")
  ws1 = wb.get_sheet_by_name("Sheet1")
  
  #This works:
  print ws1.cell(row=1, column=1).value 
  
  #This doesn't work:
  ws1['A2'] = "SomeValue1"
  
  #This doesn't work either:
  ws1.cell(row=3, column=1).value = "SomeValue2"
  </code></pre>
  
  <p>I am sure the code is correct ... What is going wrong here? </p>