Python object history

Question

I am trying to come up with a way to keep track of various re-incarnations of an object in Python. For example, I would like to have a class for, say, a room. This room could start its existence as a 'kitchen', and then, at any given point in time, become a 'study'. I want to be able to track (or log) all these various phases in the life of my room. So, I came up with this, and am wondering if there is a better way to do this. Or maybe I'm missing something. From what I can tell, though, it appears to work for my purposes.

class room():
    def __init__(self,current="Any", history = []):
        self.current = childroom(current)
        self.history = history
        self.history.append(self.current)

    def changeroom(self,current = "New"):
        self.current = childroom(current)
        self.history.append(self.current)


class childroom(room):
    def __init__(self,name="Any"):
        self.name = name    

When I use this code...

>>> myroom = room("kitchen")
>>> myroom.changeroom("study")
>>> myroom
<__main__.room instance at 0x0000000003139208>
>>> myroom.history
[<__main__.childroom instance at 0x0000000003139288>, <__main__.childroom instance at 0x00000000023F4AC8>]
>>> myroom.history[0].name
'kitchen'

Answer 1

I personally would implement it like this:

#! /usr/bin/python3
import copy

class Room:
    def __init__ (self, state = 'Initial'):
        self.state = state
        self.history = []

    def morph (self, state):
        clone = copy.deepcopy (self)
        self.state = state
        self.history.append (clone)

Keep in mind, that I don't know if your real setup has some features that restrict deep copying.

This yields:

>>> r = Room ('Kitchen')
>>> r.morph ('Loo')
>>> r.morph ('Spaceship')
>>> r.state
'Spaceship'
>>> [a.state for a in r.history]
['Kitchen', 'Loo']
>>> [type (a) for a in r.history]
[<class 'test.Room'>, <class 'test.Room'>]

---

I guess normally you don't need to save the whole state of an object, but only attributes which are worth tracking. You could pack this behaviour into a decorator along these lines:

#! /usr/bin/python3

import datetime
import copy

class Track:
    def __init__ (self, *args, saveState = False):
        self.attrs = args
        self.saveState = saveState

    def __call__ (self, cls):
        cls._track = []
        this = self

        oGetter = cls.__getattribute__
        def getter (self, attr):
            if attr == 'track': return self._track
            if attr == 'trackTrace': return '\n'.join ('{}: "{}" has changed to "{}"'.format (*t) for t in self._track)
            return oGetter (self, attr)
        cls.__getattribute__ = getter

        oSetter = cls.__setattr__
        def setter (self, attr, value):
            if attr in this.attrs:
                self._track.append ( (datetime.datetime.now (), attr, copy.deepcopy (value) if this.saveState else value) )
            return oSetter (self, attr, value)
        cls.__setattr__ = setter

        return cls

Now we can use this decorator like this:

@Track ('holder')
class Book:
    def __init__ (self, title):
        self.title = title
        self.holder = None
        self.price = 8

class Person:
    def __init__ (self, firstName, lastName):
        self.firstName = firstName
        self.lastName = lastName

    def __str__ (self):
        return '{} {}'.format (self.firstName, self.lastName)

r = Book ('The Hitchhiker\'s Guide to the Galaxy')
p = Person ('Pedro', 'Párramo')
q = Person ('María', 'Del Carmen')
r.holder = p
r.price = 12
r.holder = q
q.lastName = 'Del Carmen Orozco'
print (r.trackTrace)

If called with @Track ('holder'), it yields:

2013-10-01 14:02:43.748855: "holder" has changed to "None"
2013-10-01 14:02:43.748930: "holder" has changed to "Pedro Párramo"
2013-10-01 14:02:43.748938: "holder" has changed to "María Del Carmen Orozco"

If called with @Track ('holder', 'price'), it yields:

2013-10-01 14:05:59.433086: "holder" has changed to "None"
2013-10-01 14:05:59.433133: "price" has changed to "8"
2013-10-01 14:05:59.433142: "holder" has changed to "Pedro Párramo"
2013-10-01 14:05:59.433147: "price" has changed to "12"
2013-10-01 14:05:59.433151: "holder" has changed to "María Del Carmen Orozco"

If called with @Track ('holder', saveState = True), it yields:

2013-10-01 14:06:36.815674: "holder" has changed to "None"
2013-10-01 14:06:36.815710: "holder" has changed to "Pedro Párramo"
2013-10-01 14:06:36.815779: "holder" has changed to "María Del Carmen"

Answer 2

This sounds like a case for a facade pattern - have an inner room object which captures current state, and use __setattr__ on the facade to capture assignments, archive the previous state (inner object), and create a new one with the updated property value. You'll want to override __getattr__ to delegate to the inner object, which will also have the appropriate methods for anything else.