def _subtree_last_position(self, p):
    """返回树的最右节点"""
    walk = p
    while self.right(walk) is not None:
        walk = self.right(walk)
    return walk

下面是一些公开的访问方法：

def first(self):
    return self._subtree_first_position(
        self.root()) if len(self) > 0 else None

def last(self):
    return self._subtree_last_position(
        self.root()) if len(self) > 0 else None

def before(self, p):
    """前驱位置"""
    if self.left(p):
        return self._subtree_last_position(self.left(p))
    else:
        walk = p
        above = self.parent(walk)
        while above is not None and walk == self.left(above):
            walk = above
            above = self.parent(walk)
        return above

def after(self, p):
    """后继位置"""
    if self.right(p):
        return self._subtree_first_position(self.right(p))
    else:
        walk = p
        above = self.parent(walk)
        while above is not None and walk == self.right(above):
            walk = above
            above = self.parent(walk)
        return above

def find_position(self,k):
    if self.is_empty():
        return None
    else:
        p = self._subtree_search(self.root(),k)
        return p

接下来是映射的核心方法：

def __getitem__(self, k):
    if self.is_empty():
        raise KeyError('Key Error'+repr(k))
    else:
        p = self._subtree_search(self.root(),k)
        if k!=p.key():
            raise KeyError('Key Error' + repr(k))
        return p.value()

def __setitem__(self, k,v):
    if self.is_empty():
        leaf = self.add_root(self._Item(k,v))
    else:
        p = self._subtree_search(self.root(),k)
        if p.key() == k:
            p.element().value = v
            return
        else:
            item = self._Item(k,v)
            if p.key() < k:
                leaf = self.add_right(p,item)
            else:
                leaf = self.add_left(p, item)

def __iter__(self):
    p = self.first()
    while p is not None:
        yield p.key()
        p = self.after(p)