[Python-Dev] Evil setattr hack
Guido van Rossum
Sat, 19 Apr 2003 13:22:57 -0400
> This post inspired me to check the way new-style class instances work
> with properties. Running the following code will demonstrate that
> although the __setattr__ hack is blocked, you can still access the
> instance's dict. This can obviously be fixed by using __slots__, but
> that seems unwieldy. Should we do anything?
> class C(object):
> def _getx(self):
> print "getting x:", self._x
> return self._x
> def _setx(self, value):
> print "setting x with:", value
> self._x = value
> x = property(_getx, _setx)
> a = C()
> a.x = 1
> object.__setattr__(a, 'x', 'foo')
> a.__dict__['x'] = 'spam'
> print a.__dict__['x']
I see nothing wrong with that. It falls in the category "don't do
that", but I don't see why we should try to make it impossible.
The thing with attributes of built-in types was different. This can
affect multiple interpreters, which is evil. It also is too
attractive to expect people not to use it if it works (since many
people *think* they have a need to modify built-in types). That's why
I go to extra lengths to make it impossible, not just hard.
--Guido van Rossum (home page: http://www.python.org/~guido/)