On Mon, 13 Jul 2009 14:20:13 -0700, Aaron Scott wrote: >> BTW, you should derive all your classes from something. If nothing >> else, use object. >> class textfile(object): > > Just out of curiousity... why is that? I've been coding in Python for a > long time, and I never derive my base classes. What's the advantage to > deriving them?
"Old style" classes (those whose base classes aren't derived from anything) have a few disadvantages: (1) Properties don't work correctly: >>> class Parrot: # old-style class ... def __init__(self): ... self._x = 3 ... def _setter(self, value): ... self._x = value ... def _getter(self): ... print "Processing ..." ... return self._x + 1 ... x = property(_getter, _setter) ... >>> p = Parrot() >>> p.x Processing ... 4 >>> p.x = 2 >>> p.x 2 In general, anything that uses the descriptor protocol, not just property, will fail to work correctly with old-style classes. (2) Classes using multiple inheritance with diamond-shaped inheritance will be broken. (3) __slots__ is just an attribute. (4) super() doesn't work. And, depending on whether you consider this a disadvantage or an advantage: (5) Special methods like __len__ can be over-ridden on the instance, not just the class: >>> class K: ... def __len__(self): ... return 0 ... >>> k = K() >>> len(k) 0 >>> k.__len__ = lambda : 42 >>> len(k) 42 In their favour: (1) Less typing. (2) If you're not using descriptors, including property(), or multiple inheritance with diamond diagrams, they work fine. (3) They're (apparently) a tiny bit faster. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
