title: Cannot append string to dictionary key
body: <p>I've been programming for less than four weeks and have run into a problem that I cannot figure out. I'm trying to append a string value to an existing key with an existing string stored in it but if any value already exists in the key I get "str object has no attribute 'append'. </p>
  
  <p>I've tried turning the value to list but this also does not work. I need to use the .append() attribute because update simply replaces the value in clientKey instead of appending to whatever value is already stored. After doing some more research, I understand now that I need to somehow split the value stored in clientKey. </p>
  
  <p>Any help would be greatly appreciated.</p>
  
  <pre><code>data = {}
  
  while True:
  
      clientKey = input().upper()
      refDate = strftime("%Y%m%d%H%M%S", gmtime())
      refDate = refDate[2 : ]
      ref = clientKey + refDate
  
      if clientKey not in data:
          data[clientKey] = ref
  
      elif ref in data[clientKey]:
          print("That invoice already exists")
  
      else:
          data[clientKey].append(ref)
  
          break
  </code></pre>