On Dec 17, 2007 11:48 AM, Steven Clark <[EMAIL PROTECTED]> wrote: > Hi all- > I was reading http://dirtsimple.org/2004/12/python-is-not-java.html, in > particular the part about "getters and setters are evil": > "In Java, you have to use getters and setters because using public fields > gives you no opportunity to go back and change your mind later to using > getters and setters. So in Java, you might as well get the chore out of the > way up front. In Python, this is silly, because you can start with a normal > attribute and change your mind at any time, without affecting any clients of > the class. So, don't write getters and setters." > > I understand the idea behind this, but how does this transition work in > actuality? > Lets say I define class Foo initially as... > > class Foo(object): > def __init__(self, x): > self.x = x > > def get_sqrt(self): > return sqrt(self.x) > > Fine. I can read from x, and write to x: > def test(): > f = Foo(9) > print 'sqrt of', f.x, 'is', f.get_sqrt() > f.x = 16 > print 'sqrt of', f.x, 'is', f.get_sqrt() > > Let's say later on I decide that f.get_sqrt will be called far more often > than f.x is changed, and that it therefore makes sense to store the actual > sqrt in the class, rather than calling math.sqrt each time f.get_sqrt is > called. So I rewrite Foo: > > class Foo(object): > def __init__(self, x): > self._x = x > self._my_root = sqrt(x) > > def _set_x(self, x): > self._x = x > self._my_root = sqrt(x) > > def get_sqrt(self): > return self._my_root > > x = property(lambda self: self._x, _set_x) > > External to the class, everything behaves as before, except with the benefit > of not having to wait for slow sqrt each time. self.x is now a property > rather than an attribute. Internal to the class, I have to search & replace > all old occurences of "self.x" with "self._x"? Otherwise there is a > collision between f.x the attribute and f.x the property? >
You only need to update uses of the attribute that *need* the raw attribute, and not the property. Quite often these are just your (new) getters and setters, as you see. There is no "collision" - properties and (raw) attributes are accessed the same way. Any use of f.x will go through the property. -- http://mail.python.org/mailman/listinfo/python-list
