title: Openpyxl get_column_letter ValueError
body: <p>I am making an excel comparing tool in python. Here is my code:</p>
  
  <p>bananacell.py:</p>
  
  <pre><code>import time
  
  print("Welcome to Banana Cell the open source excel tool!")
  time.sleep(1)
  import compare
  compare.main()
  </code></pre>
  
  <p>compare.py:</p>
  
  <pre><code>import openpyxl, tkinter, time
  from tkinter import Tk
  from tkinter.filedialog import askopenfilename
  from openpyxl.utils import get_column_letter
  
  
  def main():
      print('Select file 1 from folder')
      root = Tk()
      filename = askopenfilename()
      root.withdraw()
      wb1 = openpyxl.load_workbook(filename)
      names1 = input('Sheet 1 name: ')
      sheet1 = wb1.get_sheet_by_name(str(names1))
  
      print('Select file 2 from folder')
      root1 = Tk()
      filename1 = askopenfilename()
      root1.withdraw()
      wb2 = openpyxl.load_workbook(str(filename1))
      names2 = input('Sheet 2 name: ')
      sheet2 = wb2.get_sheet_by_name(str(names2))
  
      object1 = list(sheet1['A1':get_column_letter(sheet1.max_row)])
      object2 = list(sheet2['A1':get_column_letter(sheet2.max_row)])
  
      for i in object1:
          for x in object2:
              if i != x:
                  print('Diff found!')
                  print (i.value, i.coordinate)
                  print (x.value, x.coordinate)
                  print('----------')
                  break
  </code></pre>
  
  <p>But I keep on getting this following ValueError:</p>
  
  <p><a href="https://i.stack.imgur.com/P5qHf.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/P5qHf.png" alt="enter image description here"></a></p>
  
  <p>Does anyone know how to fix this? Any help would be appreciated.</p>