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Re: Creating an object that can track when its attributes are modified

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Hi,
maybe you could do this by a decorator on the setattr method. It should 
look more or less
like your implementation, but in my eyes it's a cleaner and can be reused.

Further, I would use a stack for each attribute, so that you can restore 
all previous values.

bg,
Johannes

On 03/06/2013 05:07 PM, Ben Sizer wrote:
> I am trying to make an object that can track when its attributes have been assigned new values, and which can rollback to previous values where necessary. I have the following code which I believe works, but would like to know if there are simpler ways to achieve this goal, or if there are any bugs I haven't seen yet.
>
>
> class ChangeTrackingObject(object):
>      def __init__(self):
>          self.clean()
>
>      def clean(self):
>          """Mark all attributes as unmodified."""
>          object.__setattr__(self, '_dirty_attributes', dict())
>
>      def dirty_vals(self):
>          """Returns all dirty values."""
>          return dict( [ (k,v) for k,v in self.__dict__.iteritems() if k in self._dirty_attributes]  )
>
>      def get_changes_and_clean(self):
>          """Helper that collects all the changes and returns them, cleaning the dirty flags at the same time."""
>          changes = self.dirty_vals()
>          self.clean()
>          return changes
>
>      def rollback(self):
>          """Reset attributes to their previous values."""
>          for k,v in self._dirty_attributes.iteritems():
>              object.__setattr__(self, k, v)
>          self.clean()
>
>      def __setattr__(self, key, value):
>          # If the first modification to this attribute, store the old value
>          if key not in self._dirty_attributes:
>              if key in self.__dict__:
>                  self._dirty_attributes[key] = object.__getattribute__(self, key)
>              else:
>                  self._dirty_attributes[key] = None
>          # Set the new value
>          object.__setattr__(self, key, value)
>
>
> I am aware that adding a new attribute and then calling rollback() leaves the new attribute in place with a None value - maybe I can use a special DeleteMe marker object in the _dirty_attributes dict along with a loop that calls delattr on any attribute that has that value after a rollback.
>
> I also believe that this won't catch modification to existing attributes as opposed to assignments: eg. if one of the attributes is a list and I append to it, this system won't notice. Is that something I can rectify easily?
>
> Any other comments or suggestions?
>
> Thanks,

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Creating an object that can track when its attributes are modified Ben Sizer <kylotan@gmail.com> - 2013-03-06 08:07 -0800
  Re: Creating an object that can track when its attributes are modified Chris Angelico <rosuav@gmail.com> - 2013-03-07 03:22 +1100
    Re: Creating an object that can track when its attributes are modified Ben Sizer <kylotan@gmail.com> - 2013-03-06 08:56 -0800
      Re: Creating an object that can track when its attributes are modified Chris Angelico <rosuav@gmail.com> - 2013-03-07 04:03 +1100
      Re: Creating an object that can track when its attributes are modified 88888 Dihedral <dihedral88888@googlemail.com> - 2013-03-06 13:07 -0800
      Re: Creating an object that can track when its attributes are modified 88888 Dihedral <dihedral88888@googlemail.com> - 2013-03-06 13:07 -0800
    Re: Creating an object that can track when its attributes are modified Ben Sizer <kylotan@gmail.com> - 2013-03-06 08:56 -0800
      Re: Creating an object that can track when its attributes are modified Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-03-07 00:07 +0000
        Re: Creating an object that can track when its attributes are modified Ben Sizer <kylotan@gmail.com> - 2013-03-06 16:26 -0800
          Re: Creating an object that can track when its attributes are modified Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-03-08 18:50 +0000
  Re: Creating an object that can track when its attributes are modified Lele Gaifax <lele@metapensiero.it> - 2013-03-06 17:56 +0100
  Re: Creating an object that can track when its attributes are modified Schneider <js@globe.de> - 2013-03-07 13:21 +0100

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