On 3 Nov 2005 12:20:35 GMT, Antoon Pardon <[EMAIL PROTECTED]> wrote: >Op 2005-11-03, Stefan Arentz schreef <[EMAIL PROTECTED]>: >> Antoon Pardon <[EMAIL PROTECTED]> writes: >> >>> Op 2005-11-03, Steven D'Aprano schreef <[EMAIL PROTECTED]>: >>> >>> >> There are two possible fixes, either by prohibiting instance variables >>> >> with the same name as class variables, which would allow any reference >>> >> to an instance of the class assign/read the value of the variable. Or >>> >> to only allow class variables to be accessed via the class name itself. >>> > >>> > There is also a third fix: understand Python's OO model, especially >>> > inheritance, so that normal behaviour no longer surprises you. >>> >>> No matter wat the OO model is, I don't think the following code >>> exhibits sane behaviour: >>> >>> class A: >>> a = 1 >>> >>> b = A() >>> b.a += 2 >>> print b.a >>> print A.a >>> >>> Which results in >>> >>> 3 >>> 1 >> >> I find it confusing at first, but I do understand what happens :-) > >I understand what happens too, that doesn't make it sane behaviour. > >> But really, what should be done different here? > >I don't care what should be different. But a line with only one >referent to an object in it, shouldn't be referring to two different >objects. > >In the line: b.a += 2, the b.a should be refering to the class variable >or the object variable but not both. So either it could raise an >attribute error or add two to the class variable. > >Sure one could object to those sematics too, but IMO they are preferable >to what we have now. > A somewhat similar name space problem, where you could argue that "a" prior to += should be seen as defined in the outer scope, but lookahead determines that a is local to inner, period, so that is the reference that is used (and fails).
>>> def outer(): ... a = 1 ... def inner(): ... a += 2 ... print a ... print 'outer a', a ... inner() ... print 'outer a', a ... >>> outer() outer a 1 Traceback (most recent call last): File "<stdin>", line 1, in ? File "<stdin>", line 7, in outer File "<stdin>", line 4, in inner UnboundLocalError: local variable 'a' referenced before assignment Regards, Bengt Richter -- http://mail.python.org/mailman/listinfo/python-list
