Steven D'Aprano <[EMAIL PROTECTED]> wrote:
...
> Trying to prevent setting new attributes is a pretty heavy-handed act just
> to prevent a tiny subset of errors. Many people argue strongly that even
> if you could do it, it would be pointless -- or at least, the cost is far
> greater than whatever small benefit there is.
I entirely agree with you (also about the use of __slots__ being a
particularly WRONG way to achieve this). When I have to suggest a mixin
to avoid accidental setting of misspelled attributes (which does appear
to be a clinically certifiable phobia of programmers coming to Python
from certain other languages) I suggest something like:
class Rats(object):
def __setattr__(self, name, value):
if hasattr(self, name):
super(Rats, self).__setattr__(name, value)
else:
raise AttributeError, "can't set attribute %r" % (name,)
The key idea is to exploit hasattr's semantics -- it checks the class as
well as the specific instance.
Example use case:
class Bah(Rats):
foo = bar = baz = 23
now, given b=Bah(), you can set b.foo, b.bar and b.baz, but no other
attribute of b (of course you can bypass the restriction easily -- such
restrictions are always intended against *accidental* cases, not against
deliberate attacks).
Differently from __slots__, Rats gives no problems with pickling,
inheritance, etc, etc.
Alex
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