title: Set multiple selected items in ttk Treeview
body: <p>When using the Ttk Treeview control, I am getting a TclError when trying to programmatically set multiple selected items.</p>
  
  <p>What is the correct way to set multiple items as selected in a Treeview control?</p>
  
  <p><a href="https://docs.python.org/2/library/ttk.html#ttk.Treeview.selection_set" rel="nofollow noreferrer">The documentation</a> isn't clear about what types are allowed for <code>items</code>:</p>
  
  <blockquote>
    <p><strong><code>selection_set(items)</code></strong> <em>items</em> becomes the new selection.</p>
  </blockquote>
  
  <p>I've simplified my code to the following:</p>
  
  <pre><code>try:  # python 2
      import Tkinter as tk
      import ttk
  except ImportError:  # python 3
      import tkinter as tk
      from tkinter import ttk
  
  root = tk.Tk()
  tree = ttk.Treeview(root)
  for text in ['apple', 'banana', 'coconut']:
      tree.insert('', 'end', text=text)
  all_items = list(tree.get_children())
  
  print("all_items = {!r}".format(all_items))
  tree.selection_set(all_items)
  </code></pre>
  
  <p>However, it raises an exception:</p>
  
  <pre><code>all_items = ['I001', 'I002', 'I003']
  Traceback (most recent call last):
    File ...
      tree.selection_set(all_items)
    File "C:\Python27\lib\lib-tk\ttk.py", line 1402, in selection_set
      self.selection("set", items)
    File "C:\Python27\lib\lib-tk\ttk.py", line 1397, in selection
      return self.tk.call(self._w, "selection", selop, items)
  _tkinter.TclError: Item ['I001', not found
  </code></pre>
  
  <p>The last line of the error message makes it seem like it is converting the list of items to a string just using str, but the format isn't what is expected by the backend.</p>