title: NameError in Python program even though it seems to be setup correctly
body: <p>I copied this program from the Mission Python book:</p>
  
  <pre><code>room_map = [[1,1,1,1,1],
      [1,0,0,0,1],
      [1,0,1,0,1],
      [1,0,0,0,1],
      [1,0,0,0,1],
      [1,0,0,0,1],
      [1,1,1,1,1]]
  
  WIDTH = 800 # window size
  HEIGHT = 800
  
  top_left_x = 100
  top_left_y = 150
  
  DEMO_OBJECTS = [images.floor, images.pillar]
  
  room_height = 7
  room_width = 5
  
  def draw():
      for y in range(room_height):
          for x in range(room_width):
              image_to_draw = DEMO_OBJECTS[room_map[y][x]]
              screen.blit(image_to_draw,
                          (top_left_x + (x * 30),
                          top_left_y + (y * 30) - image_to_draw.get_height()))
  </code></pre>
  
  <p>When I run it I get this error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "/home/pi/escape/listing3-5.py", line 15, in &lt;module&gt;
      DEMO_OBJECTS = [images.floor, images.pillar]
  NameError: name 'images' is not defined
  </code></pre>
  
  <p>I think maybe I should import Images or some similar package but the book doesn't mention doing that so I'm not sure. The python file is in the correct location and the images folder containing my images is in the same folder. Have I missed anything obvious?</p>