import copy

def Memento(obj, deep=False):
   state = (copy.copy, copy.deepcopy)[bool(deep)](obj.__dict__)
   def Restore():
      obj.__dict__.clear()
      obj.__dict__.update(state)
   return Restore

class Transaction:
   """A transaction guard. This is realy just 
      syntactic suggar arount a memento closure.
   """
   deep = False
   def __init__(self, *targets):
      self.targets = targets
      self.Commit()
   def Commit(self):
      self.states = [Memento(target, self.deep) for target in self.targets]
   def Rollback(self):
      for state in self.states:
         state()

class transactional(object):
   """Adds transactional semantics to methods. Methods decorated 
      with @transactional will rollback to entry state upon exceptions.
   """
   def __init__(self, method):
      self.method = method
   def __get__(self, obj, T):
      def transaction(*args, **kwargs):
         state = Memento(obj)
         try:
            return self.method(obj, *args, **kwargs)
         except:
            state()
            raise
      return transaction

if __name__ == '__main__':

   class NumObj(object):
      def __init__(self, value):
         self.value = value
      def __repr__(self):
         return '<%s: %r>' % (self.__class__.__name__, self.value)
      def Increment(self):
         self.value += 1
      @transactional
      def DoStuff(self):
         self.value = '1111' # <- invalid value
         self.Increment()    # <- will fail and rollback

   print
   n = NumObj(-1)
   print n
   t = Transaction(n)
   try:
      for i in range(3):
         n.Increment()
         print n
      t.Commit()
      print '-- commited'
      for i in range(3):
         n.Increment()
         print n
      n.value += 'x' # will fail
      print n
   except:
      t.Rollback()
      print '-- rolled back'
   print n
   print '-- now doing stuff ...'
   try:
      n.DoStuff()
   except:
      print '-> doing stuff failed!'
      import traceback
      traceback.print_exc(0)
      pass
   print n