title: Assistance with TypeError:write() argument must be str, not None
body: <p>My code takes a regular paragraph file and switches it up to ROT13 characters.<br>
  I keep getting the error ("TypeError: write() argument must be str, not None") and have no idea why/what it's talking about.<br>
  My code works fine outputting everything correctly into the file, but this error is really bugging me. </p>
  
  <p>Function</p>
  
  <pre><code># Constants
  ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
  ROT13_ALPHABET = "NOPQRSTUVWXYZABCDEFGHIJKLM"
  
  
  def code_ROT13(file_variable_input, file_variable_output):
      line = file_variable_input.readline().strip()
      alphabet_list = list(ALPHABET)
      rot13_list = list(ROT13_ALPHABET)
  
      while line != "":
          for letter in line:
              rot13_edit = False
              for i in range(len(alphabet_list)):
                  if letter == alphabet_list[i] and not rot13_edit:
                      letter = rot13_list[i]
                      rot13_edit = True
                  elif letter == alphabet_list[i].lower() and not rot13_edit:
                      letter = rot13_list[i].lower()
                      rot13_edit = True
              file_variable_output.write(letter)
  
          line = file_variable_input.readline()
  </code></pre>
  
  <p>Main</p>
  
  <pre><code># Imports
  from cipher import code_ROT13
  # Inputs
  file_name_input = input("Enter input file name: ")
  file_name_input = "code.txt"
  file_variable_input = open(file_name_input, "r")
  
  file_name_output = input("Enter output file name: ")
  file_name_output = "code_ROT13.txt"
  file_variable_output = open(file_name_output, "w")
  
  # Outputs
  editversion = code_ROT13(file_variable_input, file_variable_output)
  file_variable_output.write(editversion)
  
  file_name_input.close()
  </code></pre>