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.
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