如何在 Python 中为列表类实现 undo() 方法

Question

我对 python 有点新，我有一个任务来创建一个带有 undo() 方法的类“UndoList”（类型列表）。 此方法应撤消典型的列表操作，如追加、插入、删除...

>>> ul = UndoList([1,2,3])
>>> ul.append(4), print(ul), undo(ul), print(ul)
[1,2,3,4]
[1,2,3]
>>> ul.remove(3), print(ul), undo(ul), print(ul)
[1,2]
[1,2,3]
...

这个 undo() 方法应该只撤销一个操作（如你在示例中看到的）。 我的老师给了我提示，在每次操作之前将列表的值保存在实例中。

这是我的课：

class UndoList(list):

   def __init__(self, lis):
       list.__init__(self, lis)
       self.lis = []

   def __append__(self, lis):
       list.__add__(self, lis)
       return lis

   def undo(self):
       return self

a1 = UndoList([1,2,3])
print(a1), a1.append(4), print(a1)   #[1,2,3] [1,2,3,4]
a1.undo(), print(a1)                 #[1,2,3,4]

所以现在我的问题是：如何在我的类中创建一个实例以在我执行任何操作之前保存我的实际列表？ 是否可以在我的撤销方法中重新运行这个实例？

谢谢！

Answer 1

下面是一些可以帮助您入门的代码。 但实际上，最好避免对 Python 的标准类型进行子类化，因为要正确执行此操作，您通常需要覆盖每个方法，这可能相当乏味且容易出错。

请注意， append方法称为append ，而不是__append__ 。 :) 并且就地改变列表的方法返回None ，而不是列表。

from copy import deepcopy

class UndoList(list):
    def __init__(self, *args):
        super().__init__(*args)
        self.old = []

    def append(self, item):
        self.old = deepcopy(self[:])
        super().append(item)

    def extend(self, items):
        self.old = deepcopy(self[:])
        super().extend(items)

    def undo(self):
        temp = deepcopy(self[:])
        self[:] = self.old
        self.old = temp


a = UndoList([1, 2, 3])
print(a)

a.append(4)
print(a)
a.undo()
print(a)
a.undo()
print(a)

a.extend([5, 6])
print(a)
a.undo()
print(a)

输出

[1, 2, 3]
[1, 2, 3, 4]
[1, 2, 3]
[1, 2, 3, 4]
[1, 2, 3, 4, 5, 6]
[1, 2, 3, 4]

我们使用def __init__(self, *args)以便我们可以在没有参数的情况下调用UndoList()来获得一个空的 UndoList。

正如评论中提到的 9000，您可能不需要在这里进行deepcopy 。 它通过递归复制每个列表项（不可变项除外）来消耗额外的 RAM，而且速度很慢。 使用deepcopy确实使UndoList健壮。 OTOH，这也意味着从.old恢复的项目是原始项目的副本，在某些情况下这是不可取的 - 如果其他对象引用这些项目，则备份过程会中断该连接。

如果您想对此进行试验，只需将备份列表的代码更改为

self.old = self[:]

undo方法变为

def undo(self):
    self[:], self.old = self.old, self[:]

---

这样做的明智方法是使用Abstract Base Classes而不是 sub-classing list来构建一个新类。

Answer 2

这很简单。 但是很乏味：你向列表对象添加了一个history属性。 这是先前变量状态的堆栈。 每个更改操作都需要在更改之前将其当前状态推送到对象的history 。 undo操作只是弹出最近的一个。

您已经注意到您必须重新定义所有更改操作（例如类中的__append__ ）。

Answer 3

让我们通过例子来理解：最初给定一个空数组，我们必须实现四个函数。

我们有以下类型的查询：

1. Add(value) ：将值添加到数组中。

2. Remove(value) ：从数组中删除值。

3. 撤消：撤消对阵列执行的最后一个操作。

4. 重做：恢复最近在阵列上执行的 UNDO 操作。

示例： 输入：List=[] 输出：add(1) print(List) List=[1]

          add(2)  print(List)  List=[1,2]
          add(3)  print(List) List=[1,2,3] 
          add(4)  print(List)  List=[1,2,3,4]
          undo()  print(List) List=[1,2,3]
          undo()  print(List)  List=[1,2]
          redo()   print(List)  List=[1,2,3]
          redo()  print(List) List=[1,2,3,4]
          remove(3) print(List)  List=[1,2,4]
          remove(1)  print(List) List=[2,4]
          remove(2)  print(List)  List=[4]
          undo() print(List)  List=[2,4]
          undo() print(List)  List=[1,2,4]
          undo() print(List)  List=[1,2,3,4]
          redo()  print(List)  List=[1,2,4]

方法：这个问题可以通过使用 2 个列表来解决，首先，我们将创建一个 undo 列表，该列表将跟踪最后执行的操作，它将是一个元组列表，格式为（操作，索引，值) 如果 operation='a' 表示执行的最后一个动作是将元素添加到列表中，并且如果 operation='r' 表示执行的最后一个动作是从列表中删除元素。

第二个列表将是重做列表，它将跟踪执行的撤消操作，以便它可以恢复最近的撤消操作。

 # Python Program to implement the above approach. # this is the main list which will be printed after doing any of the above operations main_list = [] # this is the list for tracking the operations being performed undo_list = [] # this is the redo list which will keep track of the undo operations done. redo_list = [] \\ def add(value): """ this is the function to add the value to the list """ # index at will we will add the value idx=len(main_list) # value will be added to the main_list main_list.append(value) # we will update undo_list, by appending the operation as 'r', as we are adding value to the list, so its undo operation will do the reverse of it, so we will append operation as 'r'. undo_list.append(('r',idx,value)) print(main_list) def remove(value): """ this is the function to remove the value from the list """ # length of the main_list length=len(main_list) # if the length of the main_list is 0 if(length==0): return # if the value is not present in the main_list if value not in main_list: return # index of the value that we have to remove idx = main_list.index(value) # removing value from the main_list main_list.remove(value) # we will update undo_list, by appending the operation as 'a', as we are removing value from the list , so its undo operation will do the reverse of it , so we will append operation as 'a'. undo_list.append(('a', idx, value)) print(main_list) def undo(): """ this is the function to undo the value """ #length of the undo_list length = len(undo_list) # if the length of the undo_list is 0 ,means there is nothing to do undo operation if(length==0): return # selecting the latest undo operation that we have to perform cur_tuple=undo_list.pop(); # selecting the type of the operation that we have to perform cur_tuple_operation=cur_tuple[0] # selecting the index at which we will perform latest undo operation. cur_tuple_index=cur_tuple[1] # selecting the value on which we will perform the latest undo operation cur_tuple_value=cur_tuple[2] # if the operation we have to do undo is 'a' if(cur_tuple_operation=='a'): # adding value to main_list main_list.insert(cur_tuple_index,cur_tuple_value) # also we will update redo_list by appending the operaion as 'r' as the undo current operation is 'a' , so redo operation will restore the most recent undo operation beging performed. redo_list.append(('r',cur_tuple_index,cur_tuple_value)) # if the operation we have to do undo is 'r' elif(cur_tuple_operation=='r') : # removing the value from the main_list main_list.pop(cur_tuple_index) # also we will update redo_list,by appending the operation as 'a', as the undo current operation is 'r' , so redo operation will restore the most recent undo operation beging performed. redo_list.append(('a',cur_tuple_index,cur_tuple_value)) print(main_list) def redo(): """ this is the function to redo the value """ #length of the redo_list length=len(redo_list) # if the length of the redo list is 0 if(length==0): return # selecting the latest redo operation that we have to perform. cur_tuple=redo_list.pop(); # selecting the type of the operation that we have to perform cur_tuple_operation=cur_tuple[0] # selecting the index at which we will perform latest redo operation. cur_tuple_index=cur_tuple[1] # selecting the value on which we will perform the latest redo operation. cur_tuple_value=cur_tuple[2] # if the operation we have to do redo is 'a' if(cur_tuple_operation=='a'): # adding value to main_list main_list.insert(cur_tuple_index,cur_tuple_value) # if the operation we have to do redo is 'r' elif(cur_tuple_operation=='r'): # removing the value from the main_list main_list.pop(cur_tuple_index) print(main_list) add(1) add(2) add(3) remove(2) add(4) add(5) undo() undo() undo() undo() undo() undo() redo() redo() redo() redo() redo() redo()