python traced class

With metaclass programming in python you can easily created a class tracing all updates to its attributes:

	########################################################################
	# $HeadURL $
	# File: Traced.py
	# Author: Krzysztof.Ciba@NOSPAMgmail.com
	# Date: 2012/08/08 13:29:18
	########################################################################
	""" :mod: Traced
	============
	.. module: Traced
	:synopsis: watched mutable metaclass
	.. moduleauthor:: Krzysztof.Ciba@NOSPAMgmail.com
	watched mutable metaclass tracing all updated indexes or keys
	"""
	##
	# @file Traced.py
	# @author Krzysztof.Ciba@NOSPAMgmail.com
	# @date 2012/08/08 13:29:27
	# @brief Definition of Traced metaclass.
	########################################################################
	class Traced( type ):
	"""
	.. class:: Traced
	metaclass telling if some attrs was updated
	1. overwrites __setattr__ and __setitem__
	2. adds updated member function and __updated__ attribute
	"""
	def __new__( cls, cls_name, bases, classdict ):
	""" prepare new class instance """
	def updated( self, element=None, reset=False ):
	""" updates and returns __updated__ list
	:param self: self reference
	:param mixed element: key name or list index
	:param bool reset: flag to zero __updated__ list
	:return: __updated__ list when called without arguments
	"""
	if not self.__update__ or reset:
	self.__update__ = list()
	if element and element not in self.__update__:
	self.__update__.append( element )
	return self.__update__
	def trace_setattr( self, name, value ):
	""" __setattr__ tracing value update """
	if name != "__update__":
	if not hasattr( self, name ) or getattr( self, name ) != value:
	self.updated( name )
	bases[0].__setattr__( self, name, value )
	def trace_setitem( self, ind, item ):
	""" __setitem__ tracing value update """
	try:
	if bases[0].__getitem__( self, ind ) != item:
	self.updated( ind )
	except KeyError:
	self.updated( ind )
	bases[0].__setitem__( self, ind, item )
	classdict["__setattr__"] = trace_setattr
	classdict["__setitem__"] = trace_setitem
	classdict["updated"] = updated
	classdict["__update__"] = None
	return type.__new__( cls, cls_name, bases, classdict )
	class TracedDict(dict):
	""" traced dict """
	__metaclass__ = Traced
	class TracedList(list):
	""" traced list """
	__metaclass__ = Traced
	class TracedTest( object )
	""" test class
	:param classAttribute: name says it all
	:param instanceAttribute: ditto
	"""
	__metaclass__ = Traced
	classAttribute = None
	def __init__( self ):
	self.instanceAttribute = None
	#self.updated( reset=True )

view raw Traced.py hosted with ❤ by GitHub

All objects using Traced metaclass are able to tell you which attributes have been changed. To get list of updated attributes (or keys when using TracedDict or indexes in case of TracedList) just call updated member function. There is only one shortage: attributes defined and set inside __init__ (real instance attributes like instanceAttribute in TracedTest class) will be put into updated list when calling constructor. To avoid this behavior and trace changed during lifetime of object you need at call at ther very last line in constructor self.updated( reset = True ) (or uncomment last line in above example).