CODEWITHSUNDEEP
python
gaggio
gaggio

Reputation: 545

Object attributes that are linked to others: how to maintain consistency?

Very often my classes have a few attributes, and other object's properties depend on those. What is the best way to define and access those? A minimal example should explain what I'm asking.

Say that I have a class that defines a circle:

class Circle:
    def __init__(self, r):
        self.r = r

Now this circle is instantiated just based on the radius, but I may want to be able to access its diameter, these are the possibilities I see:

A) create a new attribute when instantiating

class Circle:
    def __init__(self, r):
        self.r = r
        self.d = 2*r

B) define a function that returns the diameter

class Circle:
    def __init__(self, r):
        self.r = r
    def d(self):
        d = 2*r
        return d

Solution A does not ensure consistency, if the user changes r, d is not automatically updated, and the mistake can be hard to track down. Solution B requires the user to call a function rather than accessing an attribute. And every time the attribute d has to be accessed, it has to be recalculated, and in general it might be time consuming.

Is there a solution C that ensures consistency and does not require the class user to recalculate the attribute every time it is accessed?

Upvotes: 2

Views: 387

Answers (1)

Owen
Owen

Reputation: 1736

You could use a Python property here.

Here's a discussion of the built in property function in the docs:

http://docs.python.org/2/library/functions.html#property

In your case you could use it as follows:

def getdiameter(self):
    return self.r * 2

def setdiameter(self, d):
    self.r = d / 2

d = property(getdiameter, setdiameter)

You may want d to be readonly (it's purely a function of a radius). In this case, you can omit the setdiameter function from the property.

Update If d was very expensive to compute, then I would compute d when it was first read and invalidate d when r was computed.

def setr(self, r):
    self.r = r
    self.d = None

def getd(self):
    if not self.d is None:
        return self.d
    self.d = self.r * 2 # Our expensive calculation
    return self.d

Upvotes: 1

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