Maric Michaud a écrit :
Le Wednesday 03 September 2008 17:40:43 mk, vous avez écrit :
Note that if one property can really be computed from another, this kind
of thing could be considered as bad design (except if the computation is
heavy).
Hmm, why? Is the line of thinking smth like: because the variables
should be kept to minimum and they should be calculated at the moment
they are needed?
Because you have to make extra effort to keep the logical relation between
value and square. self._square is not really needed, and what is not needed
is just extra hassle.
Doesn't it clear that your code is more hard to maintain than the
alternative :
class Squared(object):
def __init__(self, val):
self._val=val
def fgetvalue(self):
return self._val
def fsetvalue(self, val):
self._val=val
value = property(fgetvalue, fsetvalue)
def fgetsquare(self):
return self.value ** 2
def fsetsquare(self,s):
self.value = math.sqrt(s)
square = property(fgetsquare, fsetsquare)
FWIW, if there's no computation on getting or setting value, you can
make it a plain attribute.
But while it's quite clear that in this example use case it would be
better to have only one property (weither square or value, depending on
which is the most often use), things are not always that simple in real
world code, and - as you mentionned - there may be times where you have
interdependant properties and really want to avoid recomputing the same
thing over and over. Now there's no one size fits all solution here -
it's an optimization problem, and as such depends on real use cases.
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