class atribute name mangling and exec()

Tom Harris tomh at optiscan.com
Fri Jul 27 02:14:58 CEST 2001


All,

I cannot explain this, could someone give me a simple explanation? Suppose I
have this little class:

class GreenEggs:
	def __init__(self):
		exec('self.__m1 = "I am Sam"')
		self.__m2 = 'Sam I am'
		
	def m1(self):
		print self.__m1
		
	def m2(self):
		print self.__m2
		
>>> a = GreenEggs.GreenEggs()
>>> a.m1()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "f:/dev\GreenEggs.py", line 7, in m1
    print self.__m1
AttributeError: GreenEggs instance has no attribute '_GreenEggs__m1'
>>> a.m2()
Sam I am
>>> dir (a)
['_GreenEggs__m2', '__m1']

See what I mean? The name mangling is defeated inside the evaluation
performed by the exec statement. What is the moral here, exec() is evil, or
name mangling is easily broken, so avoid it (I have a heavy C++ background,
so hiding everything in classes is a serious obsession)? 

 Tom Harris, Software Engineer
 Optiscan Imaging, 15-17 Normanby Rd, Notting Hill, Melbourne, Vic 3168,
Australia
 email tomh at optiscan.com     ph +61 3 9538 3333  fax +61 3 9562 7742

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