title: Pandas dataframe to existing excel workbook
body: <p>I'm trying to use openpyxl to write data to an existing xlsx workbook and save it as a separate file. I would like to write a dataframe to a sheet called 'Data' and write some values to another sheet called 'Summary' then save the workbook object as 'test.xlsx'. I have the following code that writes the calculations I need to the summary sheet, but I get a TypeError for an unexpected keyword argument 'font' when I try to write the dateframe, which doesn't make much sense to me... </p>
  
  <p>I'm using the following code which I've adapted from <a href="https://stackoverflow.com/questions/20219254/how-to-write-to-an-existing-excel-file-without-overwriting-data">here</a></p>
  
  <pre><code>from openpyxl import load_workbook
  import pandas as pd
  
  book = load_workbook('template.xlsx')
  
  # Write values to summary sheet
  ws = book.get_sheet_by_name('Summary')
  ws['A1'] = 'TEST'
  
  book.save('test.xlsx')
  
  # Write df 
  writer = pd.ExcelWriter('test.xlsx', engine='openpyxl') 
  writer.book = book
  writer.sheets = dict((ws.title, ws) for ws in book.worksheets)
  df.to_excel(writer, sheet_name="Data", index=False)
  writer.save()
  </code></pre>
  
  <p>The trace-back:</p>
  
  <pre><code>File "C:\Users\me\AppData\Local\Continuum\Anaconda\lib\site-    packages\pandas\core\frame.py", line 1274, in to_excel
    startrow=startrow, startcol=startcol)
  
  File "C:\Users\me\AppData\Local\Continuum\Anaconda\lib\site-packages\pandas\io\excel.py", line 778, in write_cells
    xcell.style = xcell.style.copy(**style_kwargs)
  
  File "C:\Users\me\AppData\Local\Continuum\Anaconda\lib\site-packages\openpyxl-2.2.2-py2.7.egg\openpyxl\compat\__init__.py", line 67, in new_func
    return obj(*args, **kwargs)
  
  TypeError: copy() got an unexpected keyword argument 'font'
  </code></pre>