Class Property Page 30: Possible Shortcomings?

joegal · January 27th, 2019, 10:51 AM

The circle Class seems to be designed to prevent its attribute radius from receiving negative values through the use of the property feature and a defensive setter __setRadius.

Whilst this prevents a caller from supplying a non positive value once the object has been created, it does not stop this happening when the object is created:

Code:

c1 = Circle(-30)

will set the radius to the negative value because the setter is not invoked in the initialiser.

Curiously, if you try to do this by changing

Code:

self.__radius = radius

to

Code:

self.__radius = self.__setRadius(radius)

, the interpreter does not complain but the assignment does not work with a positive value, instead radius is set to None. It does, however, work if you repeat the check / raising of ValueError for a non positive value.

I am also a little surprised that the design prevents the caller from reading the value of the radius when the property mechanism allows them to alter it, as this seems somewhat bizarre behaviour. This can be overcome by introducing a getter and altering the property assignment along the following lines:

Code:

def __getRadius(self):
        return self.__radius
    
    radius = property(__getRadius, __setRadius)

Alan G · January 27th, 2019, 01:37 PM

You are quite correct on all counts. The main issue of setting a negative radius on creation should be dealt with by raising a value error in the __init__() method if a negative value is received. The best way to do that would have been to call the property method directly from __init__(). That was careless of me.

The other points are valid but I did not think they would not have added much to the point of the exercise which was to demonstrate the value of properties. But it would have added extra code to the chapter which was only supposed to be a short refresher not a full tutorial. As an author you are constantly trying to minimise the amount of code needed to illustrate the current point. I chose to demonstrate that properties allow you to create write-only properties as well as read-only (and hopefully obviously, read-write!) But it was an entirely arbitrary choice.

Finally the reason why:

Code:

self.__radius = self.__setRadius(radius)

returns None is because None is the default return value of a Python function/method.
Instead of assigning it, you only need to call the method inside init:

Code:

def __init__(self,radius): self.__setRadius(radius)

Thanks for taklng the time to post.

joegal · January 27th, 2019, 01:45 PM

Hi Alan.

Thanks for the prompt response. I can see that __setRadius does not return a value, and I should have picked this up myself.