我终于有一些时间来弄弄这个。我被带走了。很长，但是我还是要粘贴它。我说set_item，insert，delete，find，和find_left方法，以及一些私有方法，让低级别的操作，打破了光标抽象。我还添加了一种move_cursor方法，该方法抛出IndexErrorfor索引元组超出范围或指向非顶级对象。

基本上，应该（只要）保证，只要您仅使用公共函数，光标就始终指向顶级对象，并且所有插入和删除操作都在顶级进行。从这里，你应该能够安全地实施__getitem__，__setitem__，__delitem__，等，甚至__getslice__，__setslice__。

但是，有一些皱纹。游标始终指向顶级对象的限制使其非常容易遍历嵌套列表，就好像它是平面列表一样。但这也意味着光标不能指向较低级别的对象，因此insert单独使用某些插入不可能发生。例如，假设您有三个列表：

>>> l1 = [1, 2, 3, 4]
>>> l2 = [5, 6, 7, 8]
>>> l3 = [l1, l2]
>>> l3
[[1, 2, 3, 4], [5, 6, 7, 8]]

现在，将此嵌套结构放入NLI中，移至5，然后尝试插入。

>>> nli = NestedListIter(l3)
>>> nli.find(5)
>>> nli.insert(9)
>>> nli.nested_list
[[1, 2, 3, 4], [9, 5, 6, 7, 8]]

如您所见，您可以在中插入内容l2，但不能轻松地在中插入内容l3。实际上，现在要这样做，就必须使用一个私有函数，该函数以一种不愉快的方式破坏了光标的抽象性：

>>> nli._insert_at(nli.stack[:-1], 10)
>>> nli.nested_list
[[1, 2, 3, 4], 10, [9, 5, 6, 7, 8]]
>>> nli.get_item()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "nestedlistiterator.py", line 130, in get_item
    return self._get_item_at(self.stack)
  File "nestedlistiterator.py", line 39, in _get_item_at
    item = item[i]
TypeError: 'int' object is unsubscriptable

当然，有一些方法可以实现安全的公共insert_between_branches方法，但是它们所涉及的复杂性超出了我现在所关心的范围。

当尝试在后面插入值时出现另一个问题4。如您所见，您可以在l2before之前插入一个值5，但是如果将光标移至4and
insert，则会很快意识到您不能在4inside之后插入任何内容l1。

>>> nli.go_to_head()
>>> nli.find(4)
>>> nli.insert(11)
>>> nli.nested_list
[[1, 2, 3, 11, 4], 10, [9, 5, 6, 7, 8]]

从平面访问的角度来看，在4之后插入和在5之前插入是同一回事，但是从嵌套列表的角度来看，它们是不同的。由于insert实际上是a
left_insert，所以可以使用一种right_insert方法（此方法将无法在l1的开头插入）部分纠正此问题。

通过允许光标指向较低级别的对象，可以更一般地解决这些问题，但这会使平面访问更加复杂。简而言之，纠正这些问题的任何尝试都将导致更大的复杂性，无论是在接口的平面还是嵌套的一侧。

（这实际上就是为什么我仍然偏爱简单enumerate_nested方法的原因！在所有节点（而不仅仅是顶层节点）都具有值的适当树形结构也可能会变得更简单，更好。但这仍然很有趣。）

import collections

class NestedListIter(object):
    '''A mutable container that enables flat traversal of a nested tree of 
    lists. nested_list should contain only a list-like mutable sequence. 
    To preserve a clear demarcation between 'leaves' and 'branches', empty 
    sequences are not allowed as toplevel objects.'''
    def __init__(self, nested_list):
        if not nested_list:
            raise ValueError, 'nested_list must be a non-empty sequence'
        self.nested_list = nested_list # at some point, vet this to make sure
        self.go_to_head()              # it contains no empty sequences

    def _is_sequence(self, item=None):
        '''Private method to test whether an item is a non-string sequence.
        If item is None, test current item.'''
        if item is None:
            item = self._get_item_at(self.stack)
        return isinstance(item, collections.Sequence) and not isinstance(item, basestring)

    def _is_in_range(self, index_tuple=None):
        '''Private method to test whether an index is in range. 
        If index is None, test current index.'''
        if index_tuple is None:
            index_tuple = self.stack
        if any(x < 0 for x in index_tuple):
            return False
        try:
            self._get_item_at(index_tuple)
        except IndexError:
            return False
        else:
            return True

    def _get_item_at(self, index_tuple):
        '''Private method to get item at an arbitrary index, with no bounds checking.'''
        item = self.nested_list
        for i in index_tuple:
            item = item[i]
        return item

    def _set_item_at(self, index_tuple, value):
        '''Private method to set item at an arbitrary index, with no bounds checking.
        Throws a ValueError if value is an empty non-string sequence.'''
        if self._is_sequence(value) and not value:
            raise ValueError, "Cannot set an empty list!"
        containing_list = self._get_item_at(index_tuple[:-1])
        containing_list[index_tuple[-1]] = value

    def _insert_at(self, index_tuple, value):
        '''Private method to insert item at an arbitrary index, with no bounds checking.
        Throws a ValueError if value is an empty non-string sequence.'''
        if self._is_sequence(value) and not value:
            raise ValueError, "Cannot insert an empty list!"
        containing_list = self._get_item_at(index_tuple[:-1])
        containing_list.insert(index_tuple[-1], value)

    def _delete_at(self, index_tuple):
        '''Private method to delete item at an arbitrary index, with no bounds checking.
        Recursively deletes a resulting branch of empty lists.'''
        containing_list = self._get_item_at(index_tuple[:-1])
        del containing_list[index_tuple[-1]]
        if not self._get_item_at(index_tuple[:-1]):
            self._delete_at(index_tuple[:-1])

    def _increment_stack(self):
        '''Private method that tires to increment the top value of the stack.
        Returns True on success, False on failure (empty stack).'''
        try:
            self.stack[-1] += 1
        except IndexError:
            return False
        else: 
            return True

    def _decrement_stack(self):
        '''Private method that tries to decrement the top value of the stack.
        Returns True on success, False on failure (empty stack).'''
        try:
            self.stack[-1] -= 1
        except IndexError:
            return False
        else:
            return True

    def go_to_head(self):
        '''Move the cursor to the head of the nested list.'''
        self.stack = []
        while self._is_sequence():
            self.stack.append(0)

    def go_to_tail(self):
        self.stack = []
        '''Move the cursor to the tail of the nested list.'''
        while self._is_sequence():
            self.stack.append(len(self.get_item()) - 1)

    def right(self):
        '''Move cursor one step right in the nested list.'''
        while self._increment_stack() and not self._is_in_range():
            self.stack.pop()
        if not self.stack:
            self.go_to_tail()
            return False
        while self._is_sequence():
            self.stack.append(0)
        return True

    def left(self):
        '''Move cursor one step left in the nested list.'''
        while self._decrement_stack() and not self._is_in_range():
            self.stack.pop()
        if not self.stack:
            self.go_to_head()
            return False
        while self._is_sequence():
            self.stack.append(len(self.get_item()) - 1)
        return True

    def move_cursor(self, index_tuple):
        '''Move cursor to the location indicated by index_tuple.
        Raises an error if index_tuple is out of range or doesn't correspond
        to a toplevel object.'''
        item = self._get_item_at(index_tuple)
        if self._is_sequence(item):
            raise IndexError, 'index_tuple must point to a toplevel object'

    def get_item(self):
        '''Get the item at the cursor location.'''
        return self._get_item_at(self.stack)

    def set_item(self, value):
        '''Set the item a the cursor locaiton.'''
        return self._set_item_at(self.stack, value)

    def insert(self, value):
        '''Insert an item at the cursor location. If value is a sequence, 
        cursor moves to the first toplevel object in value after insertion. 
        Otherwise, cursor does not move.'''
        temp_stack = self.stack[:]
        self.left()
        self._insert_at(temp_stack, value)
        self.right()

    def delete(self):
        '''Deete an item at the cursor location. Cursor does not move.'''
        temp_stack = self.stack[:]
        self.left()
        self._delete_at(temp_stack)
        self.right()

    def iterate(self):
        '''Iterate over the values in nested_list in sequence'''
        self.go_to_head()
        yield self.get_item()
        while self.right():
            yield self.get_item()

    def iterate_left(self):
        '''Iterate over the values in nested_list in reverse.'''
        self.go_to_tail()
        yield self.get_item()
        while self.left():
            yield self.get_item()

    def find(self, value):
        '''Search for value in nested_list; move cursor to first location of value.'''
        for i in self.iterate():
            if i == value:
                break

    def find_left(self, value):
        '''Search for value backwards in nested_list; move cursor to last location of value.'''
        for i in self.iterate_left():
            if i == value:
                break

def _NLI_Test():
    l = [1, 2, 3, ['a', 'b', 'c'], 4, ['d', 'e', [100, 200, 300]], 5, ['a', 'b', 'c'], 6]
    nli = NestedListIter(l)
    print nli.nested_list
    for i in nli.iterate():
        print i,
    print
    for i in nli.iterate_left():
        print i,
    print

    nli.go_to_head()
    for i in range(5):
        nli.right()
    nli.insert('cow')
    nli.insert(['c', ['o', 'w']])
    print nli.nested_list
    nli.find('cow')
    print nli.get_item()
    nli.delete()
    print nli.nested_list
    nli.find('c')
    nli.delete()
    print nli.nested_list
    nli.find_left('w')
    nli.delete()
    nli.find('o')
    nli.delete()
    print nli.nested_list
    print nli.nested_list == l
    nli.find(100)
    nli.set_item(100.1)
    print nli.nested_list

if __name__ == '__main__':
    _NLI_Test()