Currently it's possible to customize attribute access via methods : __getattribute__(self, name) __getattr__(self, name) __setattr__(self, name, value) but it is not possible to customize local variable access. It would be useful for example to allow implementation of a local constant or any other on-access/on-modification behavior. The function should could be: __getlocal__(name) __setlocal__(name, value) By default they are just access to the local dictionnary for reading and writting (AFAIK the only current way to access this dictionnary is using the locals() built-in). Here is a sample code of typical usage : ####################### #Sample 1 : Tracing local access old_getlocal = __getlocal__ old_setlocal = __setlocal__ #redefine the built-ins def __getlocal__(name): print 'accessing %s' % name return old_getlocal(name) def __setlocal__(name, value): print 'setting %s with value %s' % (name, value) old_setlocal(name, value) a = 1 b = 2 c = 3 #setting b and accessing a b = a + 1 ################### #Sample 2 : Constants old_getlocal = __getlocal__ old_setlocal = __setlocal__ class ConstantError(Exception):pass constants = [] #redefine the built-ins def __setlocal__(name, value): if old_getlocal(name, value) in constants: raise ConstantError() else: old_setlocal(name, value) a = 1 b = 2 c = 3 constants = [b, c] a = 4 #OK b = 5 #IMPOSSIBLE constant.remove(c) c = 6 #OK I found such approach more flexible than introducing a 'const' in python (not a good thing I think). And it enable many other customizations... Note : the fact that in my example I DO NOT access to any kind of writable local dictionary! This on purpose, the only way to access this dictionary should be : locals(), __getlocal__() and __setlocal__() built-ins. Pierre-Yves Martin pym<dot>aldebaran< a t >gmail<dot>com