python - Python オブジェクトに属性エラーがありません

Question

class EmptyMap():
    """
    EmptyMap has no slots
    """
    __slots__ = ()

class NonEmptyMap():
    """
    Has slots of left, key, value, and right. 
    """
    __slots__ = ('left', 'key', 'value', 'right')

def mkEmptyMap():
    """
    Is a function that takes no arguments and returns an instance of EmptyMap
    """
    return EmptyMap()

def mkNonEmptyMap(left, key, value, right):
    """
    Is a function that takes a map, a key, a value, and another map,
    and returns an instance of NonEmptyMap. This function merely initializes the slots;
    it is possible to use this function to create trees that are not binary search trees.
    """
    nonEmptyMap = NonEmptyMap()
    nonEmptyMap.left = left
    nonEmptyMap.key = key
    nonEmptyMap.value = value
    nonEmptyMap.right = right
    return nonEmptyMap

def mapInsert(key, value, node):
    """
    Is a function that takes a key, a value, and a map, and returns an instance
    of NonEmptyMap. Further, the map that is returned is a binary search tree based
    on the keys. The function inserts the key-value pair into the correct position in the
    map. The map returned need not be balanced. Before coding, review the binary
    search tree definition and the structurally recursive design pattern, and determine
    what the function should look like for maps. If the key already exists, the new value
    should replace the old value.
    """
    if isinstance(node, EmptyMap):
        return mkNonEmptyMap(mkEmptyMap(), key, value, mkEmptyMap())
    else:
        if key > node.key:
            node.right = mapInsert(key, value, node.right)
            return node.right
        elif key < node.key:
            node.left = mapInsert(key, value, node.left)
            return node.left
        elif key == node.key:
            node.value = value
            return mapInsert(key, value, node)
        else:
            raise TypeError("Bad Tree Map")

def mapToString(node):
    """
    Is a function that takes a map, and returns a string that represents the
    map. Before coding, review the structurally recursive design pattern, and determine
    how to adapt it for maps. An EmptyMap is represented as ’ ’. For an instance of
    NonEmptyMap, the left sub-tree appears on the left, and the right sub-tree appears
    on the right.
    """
    if isinstance(node, EmptyMap):
        return '_'
    elif isinstance(node, NonEmptyMap):
       return '(' + mapToString(node.left) + ',' + str(node.key) + '->'  + str(node.value) + ',' + mapToString(node.right)+ ')'
    else:
       raise TypeError("Not a Binary Tree")

def mapSearch(key, node):
    """
    Is a function that takes a key and a map, and returns the value associated
    with the key or None if the key is not there. Before coding, review the binary search
    tree definition and the structurally recursive design pattern, and determine how it
    should look for maps.
    """
    if isinstance(node, EmptyMap):
       return 'None'
    elif isinstance(node, NonEmptyMap):
       if key == node.key:
          return str(node.value)
       elif key < node.key:
          return mapSearch(key, node.left)
       elif key > node.key:
          return mapSearch(key, node.right)
    else:
       raise TypeError("Not a Binary Tree")

def main():
    smallMap = mapInsert(\
        'one',\
        1,\
        mapInsert(\
            'two',\
            2,\
            mapInsert(\
                'three',\
                3,\
                mkEmptyMap())))

    print(smallMap.key)
    print(smallMap.left.key)
    print(smallMap.right.key)

main()

プログラムを実行すると、何が間違っているのかわからない構文が得られました。emptymapにはmkNonEmptyMap関数にあるオブジェクトがあると確信しています。これは私の宿題の問題です。

マップは、値をキーに関連付けるデータ構造です。特定のキーを検索して、関連する値を見つけることができます。たとえば、値 3 はキー「three」に関連付けることができます。

one
Traceback (most recent call last):
  File "/Users/USER/Desktop/test.py", line 113, in <module>
    main()
  File "/Users/USER/Desktop/test.py", line 110, in main
    print(smallMap.left.key)
AttributeError: 'EmptyMap' object has no attribute 'key'