title: Python turtle invalid command (From Think Python book)
body: <p>I'm trying to follow along with the exercises in the "Think Python" book and ran into a problem trying to get turtle to work. <a href="http://greenteapress.com/thinkpython2/html/thinkpython2005.html" rel="nofollow">Here is the exercise I'm on. (4.1)</a></p>
  
  <p>The code they want me to execute looks like this:</p>
  
  <pre><code>import turtle
  bob = turtle.Turtle()
  print(bob)
  turtle.mainloop()
  
  bob.fd(100)
  bob.lt(90)
  bob.fd(100)
  </code></pre>
  
  <p>But when I execute it, the turtle doesn't move and I get the following error when I close the window:</p>
  
  <pre><code>Traceback (most recent call last):
      File "C:\Users\Phil\PycharmProjects\untitled\mypolygon.py", line 6, in &lt;module&gt;
          bob.fd(100)
      File "C:\Python\3.5\lib\turtle.py", line 1637, in forward
          self._go(distance)
      File "C:\Python\3.5\lib\turtle.py", line 1605, in _go
          self._goto(ende)
      File "C:\Python\3.5\lib\turtle.py", line 3158, in _goto
          screen._pointlist(self.currentLineItem),
      File "C:\Python\3.5\lib\turtle.py", line 755, in _pointlist
          cl = self.cv.coords(item)
      File "&lt;string&gt;", line 1, in coords
      File "C:\Python\3.5\lib\tkinter\__init__.py", line 2308, in coords
          self.tk.call((self._w, 'coords') + args))]
  _tkinter.TclError: invalid command name ".1395635997272"
  </code></pre>
  
  <p>I'm not sure what the debugger is saying, and I can't really continue until I figure out where I went wrong. Any help would be greatly appreciated.</p>