On Oct 17, 2:39 pm, Duncan Booth <[EMAIL PROTECTED]> wrote: > Marc 'BlackJack' Rintsch <[EMAIL PROTECTED]> wrote: > > > Simply not to introduce special cases I guess. If you write ``x.a += > > b`` then `x.a` will be rebound whether an `a.__iadd__()` exists or > > not. Otherwise one would get interesting subtle differences with > > properties for example. If `x.a` is a property that checks if the > > value satisfies some constraints ``x.a += b`` would trigger the set > > method only if there is no `__iadd__()` involved if there's no > > rebinding. > > Unfortunately that doesn't work very well. If the validation rejects the > new value the original is still modified: > > >>> class C(object): > > def setx(self, value): > if len(value)>2: > raise ValueError > self._x = value > def getx(self): > return self._x > x = property(getx, setx) > > >>> o = C() > >>> o.x = [] > >>> o.x += ['a'] > >>> o.x += ['b'] > >>> o.x += ['c'] > > Traceback (most recent call last): > File "<pyshell#27>", line 1, in <module> > o.x += ['c'] > File "<pyshell#22>", line 4, in setx > raise ValueError > ValueError > > >>> o.x > ['a', 'b', 'c']
Now that's really interesting. I added a print "before" and print "after" statement just before and after the self._x = value and these *do not get called* after the exception is raised when the third element is added. -- http://mail.python.org/mailman/listinfo/python-list
