title: Python Tkinter Canvas fail to bind keyboard
body: <p>I've been running a small script like this </p>
  
  <pre><code>from Tkinter import *
  root = Tk()
  def callback(event):
      print "callback"
  w = Canvas(root, width=300, height=300)
  w.bind("&lt;Key&gt;", callback)
  w.pack()
  root.mainloop()
  </code></pre>
  
  <p>However, the keyboard event is not handled in my situation (I use python 2.7 on window 7)</p>
  
  <p>If I use </p>
  
  <pre><code>w.bind("&lt;Button-1&gt;", callback)
  </code></pre>
  
  <p>Things work fine.</p>
  
  <p>So, this really puzzles me. 
  Please anyone tell me why this's happening, thanks in advance.</p>