Tutorial :How to build an undo storage with limit?



Question:

I want build a data structure to store limited undo buffer, take store 6 dict data for example with below pseudocode:

rawdict1 = {1}  buffer = [{1}]    rawdict1 = {2}  buffer = [{2}{1}]      # {1} stored on the postion    rawdict1 = {3}  buffer = [{3}{2}{1}]        ...  rawdict1 = {5}  buffer = [{5}{4}{3}{2}{1}]      # max length limited to 5    rawdict1 = {6}  buffer = [{6}{5}{4}{3}{2}]      # {1} has been deleted because exceed the limit    when I want to restore the rawdict1 later, I can use something looks like:    rawdict1 = buffer[5]                 # restore the 5th dict.  

My question is, can existing buildin data type or standard library type can be used for such a purpose?

And is it possible such a structure can store multi-types in one structure instance, say, if I want to store dict and self-defined class in one go?

Thanks!

Rgs,

KC


Solution:1

Perhaps use something like this:

import collections    class UndoBuffer(object):      def __init__(self,value,max_length=5):          self.max_length=max_length          self._buffer=collections.deque([value],max_length)      @property      def data(self):          return self._buffer[-1]      @data.setter      def data(self,value):          self._buffer.append(value)      def restore(self,index):          self.data=self._buffer[index]  

Make an UndoBuffer object

rawdict=UndoBuffer('{1}')        

Setting the data attribute automatically stores the value in _buffer:

print(rawdict._buffer)  # deque(['{1}'], maxlen=5)  print(rawdict.data)  # {1}  

Changing the value of rawdict.data appends the value to rawdict._buffer:

rawdict.data = '{2}'  print(rawdict._buffer)  # deque(['{1}', '{2}'], maxlen=5)  

Buf if you access rawdict.data you just get the most recent value:

print(rawdict.data)  # {2}  

Change the value a few more times. '{1}' gets dropped when the buffer is filled to its maximum length:

rawdict.data = '{3}'  rawdict.data = '{4}'  rawdict.data = '{5}'  print(rawdict._buffer)  # deque(['{1}', '{2}', '{3}', '{4}', '{5}'], maxlen=5)  rawdict.data = '{6}'  print(rawdict._buffer)  # deque(['{2}', '{3}', '{4}', '{5}', '{6}'], maxlen=5)  

Restoring the value from rawdict._buffer:

rawdict.restore(0)   # set rawdict.data to rawdict._buffer[0]  print(rawdict.data)  # {2}  print(rawdict._buffer)  # deque(['{3}', '{4}', '{5}', '{6}', '{2}'], maxlen=5)  


Solution:2

You cannot do it on a barename (such as rawdict1) because there is no way for you to intercept assignments to a barename and make them do sometime "on the side" such as saving the previous value. It's easy to do on a decorated name, e.g.:

undoable.rawdict1 = {1}  

and the like, by making undoable an instance of a class with an appropriate __setitem__ which appends the previous value (if any) to a list, and pops the 0th item if the list is getting too long. But that would not suffice for other "undoable" actions besides assignment, such as undoable.rawdict1.update(whatever) -- you sure you don't need that?


Solution:3

You can quickly subclass the list to only allow limited storage.

class LimitedStack(list):   def __init__(self,limit=6):      list.__init__(self)      self.limit = limit     def append(self,obj):      if len(self) == self.limit:          list.pop(self,0)      list.append(self,obj)  

Python lists do not have to be of a certain type like the generic lists in C#. They will store any object you append to them.


Solution:4

The collections module, as of python 2.6, contains the "deque" collection. It behaves as you need:

>>> import collections  >>> buffer = collections.deque([],6)  >>> buffer.extend(range(6))  >>> buffer  deque([0, 1, 2, 3, 4, 5], maxlen=6)  >>> buffer.append(6)  >>> buffer  deque([1, 2, 3, 4, 5, 6], maxlen=6)  >>> buffer[-1]  6  


Solution:5

Rockford Lhotka's CSLA.NET framework contains an Undo architecture. Perhaps you could study it and figure out what he did, or even use it out of the box.


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