title: Python appending not working as expected
body: <pre><code>x_list = [0, 0]
  y_list = [0, 1]
  def coordinates(condition_x, condition_y):
      zipped = zip(x_list, y_list)
      condition_coord = condition_x,condition_y
      for a,b in zipped:
          print a,b
          coord = a,b
          if condition_coord != coord:
              x_list.append(condition_x)
              y_list.append(condition_y)
          else:
              print "Already a room there"
  
  print x_list
  print y_list    
  coordinates(0,1)
  print x_list
  print y_list
  </code></pre>
  
  <p>For this code what I'm trying to accomplish is this: There are two lists <code>x_list</code> and <code>y_list</code>, and I'm checking to see if there are any coordinate pairs already there. The first index of <code>x_list</code> and <code>y_list</code> is the first coordinate, the second index of both of them is the second coordinate, etc. </p>
  
  <p>In the <code>coordinates</code> function I want to look at both of the lists, and test to see if that coordinate is already present, and if not it appends it to the 2 lists, creating another coordinate pair.</p>
  
  <p>The error that I'm getting is when it's tested, it says both "Already a room there" and appends the coordinates to the list, even though they're already there.</p>
  
  <p>Example:</p>
  
  <pre><code>coordindates(0,1)
  coordindates(0,2)
  </code></pre>
  
  <p>is supposed to return "Already a room there" for the first call, and appends 0 to X_list and 2 to y_list for the second.</p>