The "Queue" object has no "size" attribute

I've seen other examples of this event on StackOverflow, but I didn't understand any of the answers (I'm still a new programmer), and the other examples I've seen don't look similar to mine, otherwise I wouldn't "ask this question.

I am running Python 3.2 on Windows 7.

This has never happened to me before, and I have done classes many times, so this time I don't know what it really is. The only difference is that I didn't do the whole class file; I was given a template to fill and a test file to try it out. It was working on a test file but not working on my file. I am calling methods in the class exactly the same as the test file (like Lineup.size ())

This is my class:

class Queue:

    # Constructor, which creates a new empty queue:
    def __init __ (self):
        self .__ items = []

    # Adds a new item to the back of the queue, and returns nothing:
    def queue (self, item):
        self .__ items.insert (0, item)
        return

    # Removes and returns the front-most item in the queue.  
    # Returns nothing if the queue is empty.
    def dequeue (self):
        if len (self .__ items) == 0:
            return None
        else:
            return self .__ items.pop ()

    # Returns the front-most item in the queue, and DOES NOT change the queue.  
    def peek (self):
        if len (self .__ items) == 0:
            return None
        else:
            return self .__ items [(len (self .__ items) -1)]

    # Returns True if the queue is empty, and False otherwise:
    def is_empty (self):
        return len (self .__ items) == 0

    # Returns the number of items in the queue:
    def size (self):
        return len (self .__ items)

    # Removes all items from the queue, and sets the size to 0:
    def clear (self):
        del self .__ items [0: len (self .__ items)]
        return

    # Returns a string representation of the queue:
    def __str __ (self):
        return "" .join (str (i) for i in self .__ items)

This is my program:

from queue import Queue

Lineup = Queue()

while True:
  decision = str(input("Add, Serve, or Exit: ")).lower()
  if decision == "add":
    if Lineup.size() == 3:
      print("There cannot be more than three people in line.")
      continue
    else:
      person = str(input("Enter the name of the person to add: "))
      Lineup.queue(person)
      continue
  elif decision == "serve":
    if Lineup.is_empty() == True:
      print("The lineup is already empty.")
      continue
    else:
      print("%s has been served."%Lineup.peek())
      Lineup.dequeue()
      continue
  elif (decision == "exit") or (decision == "quit"):
    break
  else:
    print("%s is not a valid command.")
    continue

      

        
        
        
      

    

And this is my error message when I type "add" as my decision variable:

line 8, in builtins.AttributeError: "Queue" object has no "size" attribute

So what's going on here? What is different about this?