title: Tkinter clipboard errors
body: <p>Basically what I'm trying to do is change whatever I copy into what I copied but it's all uppercase. Whenever I try to run the following program it crashes.</p>
  
  <pre><code>from tkinter import *
  
  root = Tk()
  
  prev_clip = root.clipboard_get()
  
  while True:
      prev_clip = root.clipboard_get()
      root.clipboard_append(prev_clip.upper())
  
  root.mainloop()
  </code></pre>
  
  <p>If it's impossible to do what I want this way, then tell me how I can do it. It would be preferred if I could do it this way, though.</p>
  
  <p>Edit: This is the error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "[file location of the program]",     line 5, in &lt;module&gt;
      prev_clip = root.clipboard_get()
    File     "[file location of program]",     line 652, in clipboard_get
      return self.tk.call(('clipboard', 'get') + self._options(kw))
  _tkinter.TclError: CLIPBOARD selection doesn't exist or form "STRING" not defined
  </code></pre>