python中OBJECT的问题，求大家帮帮忙！

#!/usr/bin/env python
"""
A simple program that plays Shannon's Game.
"""

class Frequency(object):
    """
    Stores a letter:frequency pair.

    >>> f = Frequency('c', 2)
    >>> f.letter
    'c'
    >>> f.frequency
    2
    >>> f
    {c: 2}
    """
    def __init__(self, letter, frequency):
        self.letter = letter
        self.frequency = frequency
        # The next Frequency object when stored as part of a linked list
        self.next = None

    def __repr__(self):
        return '{%s: %d}' % (self.letter, self.frequency)

class FrequencyList(object):
    """Stores a collection of Frequency objects as a linked list."""
    def __init__(self):
        self.head = None

    def add(self, letter, frequency):
        """
        Adds the given `letter`:`frequency` combination as a Frequency object
        to the end of the list. If the given `letter` is already in the list, 
        the given `frequency` is added to its frequency.

        >>> f = FrequencyList()
        >>> f.add('a', 3)
        >>> f
        [{a: 3}]
        >>> f.add('b', 2)
        >>> f
        [{a: 3}, {b: 2}]
        """
        self.letter = letter
        self.frequency = frequency
        if self.head is None:
            self.head = Frequency(letter, frequency)
        else:
            current = self.head
            while current.next is not None and current.letter != letter:
                current = current.next
            if current.letter != letter:
                current.next = Frequency(letter, frequency)
            return
                
        

    def remove(self, letter):
        """
        Removes the Frequency object with the given `letter` from the list.
        Does nothing if `letter` is not in the list.

        >>> f = FrequencyList()
        >>> f.add('a', 3)
        >>> f.add('b', 2)
        >>> f.add('c', 3)
        >>> f
        [{a: 3}, {b: 2}, {c: 3}]
        >>> f.remove('b')
        >>> f
        [{a: 3}, {c: 3}]
        """
        pass